In your opinion, when does something become unforgivable?

For me, it’s really just a lack of actual remorse or effort to fix things. As long as they’re willing to demonstrate they recognize what they did and will work to be better (and it’s not a recurring pattern), we’re cool.
- I think generally the same but there are extreme cases like murder where a person can atone all they want but shouldn't necessarily be granted forgiveness.
  
  To me it kinda have to depend on what kind of murder. Accidentally killed someone in self defence or as a victim of abuse? Yeah it can be forgiven. Going out of their way to kill someone or doing something that's dangerous that result in killing someone? That's unforgivable.

Probably dishonesty. Coworker threw away thousands of dollars worth of my department's equipment and then lied to me about it. I know she was just repeating her boss, but she knew it was a lie. I'll hold a grudge against both of those bitches forever!
Two former friends hit on my husband - not just flirty, pretty seriously with an expectation of more. They're both out forever!

When they do the exact same thing after you already forgave them before.
Also when they were given plenty of warning before doing whatever they did.
So, for both cases, it's when they show that they are OK with breaking your trust because they expect to be forgiven afterwards.

If they go out of their way to hurt you, or are so indifferent about you that they hurt you in the process of doing something else and decide they don't care.
People who value you and your feelings so little that they would do either are not worth keeping around, and certainly aren't worth your effort emotionally to be able to forgive
- Totally agree on this - I have forgiven a lot of shit in my life but the one thing that has truly hurt me is when someone I cared about was offended by my methods of expressing concern to them about their behavior, and they retaliated by trying to get my partner to leave me.
  Despite the fact that it was a laughable attempt, the intention to actually hurt me, not just accidental toe-stepping or expression of frustration, is what drew a line for me.
  I will forgive unintentional damage followed by remorse, I will forgive outbursts and emotional interaction, but actively trying to hurt someone is the behavior of an adversary.

The only time something would become unforgivable is if it were done with intentional malice or becomes a pattern of behavior. I'm willing to forgive quiet a bit, if the harm an action caused was not the intent. When it becomes the intent, such as physical violence or repeated trauma, that's where I take a hard line and will not forgive. For less spectacular transgressions, repeat events are where things become unforgivable. I am willing to give grace on that more because I'm bad about not communicating to others how their actions hurt me, but if we've talked about how their behavior is a problem and they keep doing it? No sorry, we're done.

When criticism of the act gets enough likes on Lemmy and/or Reddit—that's when I know what to think.

I'm really glad you at least partially qualified what you mean by forgiveness. Or at least what you do not mean by it. The type of forgiveness I think you're trying to suggest is more of an internal process. It's not really about the other person at all. For example, there are some family members of murder victims who say they have forgiven the killer. They are not saying the killer deserves to go free. They're just saying they don't want to carry that hatred through the rest of their lives. By that definition, I don't think any of us know what we might be able to let go of.

If people are truly sorry and have taken steps to make sure that what they did will never happen again, they are eligible for forgiveness of anything.

When the same attempts to set healthy boundaries with that person are ignored repeatedly. Sure, give them a chance to try and adjust their behavior if the relationship is otherwise valuable.
But after you are sure that you have been consistent and clear for a reasonable amount of time and the boundaries still get invaded?
Bye, Felicia.

I can forgive mistakes and bad judgment, but if you intentionally do something that makes me lose trust and respect, it’s going to be hard to earn that back. If you lie to me, for example, I don’t see it as a one-time action - it reveals something about the kind of person you are. Saying you won’t do it again doesn’t mean much after you've already shown it’s something you’ve been doing up to this point.
I don’t believe in free will, so when I see someone act this way, I don’t even really blame them. I don’t see it as a conscious decision, but more as a behavior they’re helplessly repeating. And people rarely change that kind of behavior unless the motivation comes from within - not from outside pressure.
- I don’t believe in free will, so when I see someone act this way, I don’t even really blame them. I don’t see it as a conscious decision, but more as a behavior they’re helplessly repeating. And people rarely change that kind of behavior unless the motivation comes from within - not from outside pressure.
  That’s an interesting angle. But without free will, wouldn’t it be that they could never change the behavior unless it was already preordained?
  
  I don't believe people can choose to change their behavior in the way we usually think. But I do believe they can be influenced to change - by new information or experiences.
  Take this example: if we suddenly discovered that tomatoes are toxic, I wouldn’t say I’m actively choosing to stop eating them. What’s really happening is that my understanding of the world has changed so much that I automatically no longer want to eat them. It’s not about freedom of choice - it’s more like being compelled by truth. I didn’t decide to change; the change happened to me because of what I learned.

Things that betray trust are very hard for me to let go. Theft, backstabbing, lies, etc. If I can't trust you, I can't hang out with you.

Nothing.
I could cut ties with a person over the most trivial of things, but as a consequentialist I would be betraying logic by labelling somebody as unforgiveable.
That is not to say that I would ever let a person go without consequences, far from it.

It's not about "forgiveness", it's about whether the mistakes can be undone.
Consider you accidentally insult somebody. That can be undone by apologizing.
What if you murder somebody? That can never be undone.
- An apology doesn't undo what you said any more than it brings someone back to life.
- People do occasionally forgive murderers.
  Not the victims, admittedly, but their families sometimes.

That time my cousin and aunt made a scene the first night in the funeral home after my mom died. That's irreparable because
My mom died that fucking day, 2 years after my dad and they were supposed to all be close.
It was due to my personal life (my soon-to-be-wife). To add some context: she is a foreigner so there is a lot of racism and a bit of classism.
Her only living sister.
It's been 6 months and they haven't contacted me, apologized or anything,
Don't get me wrong. They are forgiven, but it hurts and it will hurt for the rest of my life so I don't be able to ever open the door for them to come back into my life.

Hmmmm probably a lot of things, but first to mind is animal cruelty
- What counts as animal cruelty?

Forgiveness is always possible, although if someone hits me with DARVO, I'm going to forever filter their behavior through a check before helping them. It's sorta amazing what we can go through and forgive after, but I guess the misnomer is that the apology is for them and not ourselves.

Granting forgiveness isn't about them, it's about you letting go of the hurt. It doesn't mean things necessarily just go back to the way they were, it means things can move onto the next thing. It's about taking that weight off your soul.
I see a lot of people letting their trauma define them. Let the shit drop and grow flowers. Easier said than done, but I think it's still easier than not doing it.

Intentionally did something dangerous that you know will get people hurt with a slightest mistake. Like driving recklessly, or pushing others in a huge crowd. There's no amount of sorry will make me believe you're remorseful, you're only sorry because stuff that will happen happened.
Ohh and embezzlement of public fund. Fuck off with that crocodile tear.

Anything done in person with absolute disrespect and malice

I am not sure - this is a great question though - at what point do you let it keep harming you instead of letting it go? All the complicated social systems of revenge and avenging and reparation built around this.
- You can forgive while also opting out of the situation, though. Forgiveness doesn't always mean a continued relationship.
  
  I don't mean letting them keep harming you - I mean when do you think it's worth keeping the hate inside you, the unforgiveness. It doesn't do anything to harm the offender, it's only harmful to the victim.
  So when is it worth holding on to that at your own expense? It's a good question.

The second time they do it. You get one chance to do some mild shit and I'll give you the benefit of the doubt. Of course, I don't announce it but I keep tabs.

Never.
Matthew 18:21-35
Then Peter came up and said to him, “Lord, how often will my brother sin against me, and I forgive him? As many as seven times?” Jesus said to him, “I do not say to you seven times, but seventy-seven times. “Therefore the kingdom of heaven may be compared to a king who wished to settle accounts with his servants. When he began to settle, one was brought to him who owed him ten thousand talents. And since he could not pay, his master ordered him to be sold, with his wife and children and all that he had, and payment to be made. So the servant fell on his knees, imploring him, ‘Have patience with me, and I will pay you everything.’ And out of pity for him, the master of that servant released him and forgave him the debt. But when that same servant went out, he found one of his fellow servants who owed him a hundred denarii, and seizing him, he began to choke him, saying, ‘Pay what you owe.’ So his fellow servant fell down and pleaded with him, ‘Have patience with me, and I will pay you.’ He refused and went and put him in prison until he should pay the debt. When his fellow servants saw what had taken place, they were greatly distressed, and they went and reported to their master all that had taken place. Then his master summoned him and said to him, ‘You wicked servant! I forgave you all that debt because you pleaded with me. And should not you have had mercy on your fellow servant, as I had mercy on you?’ And in anger his master delivered him to the jailers,until he should pay all his debt. So also my heavenly Father will do to every one of you, if you do not forgive your brother from your heart.”
- So that means everyone goes to heaven, right?
  
  Nope. The servant begged for mercy.
  So the servant fell on his knees, imploring him, ‘Have patience with me, and I will pay you everything.’ And out of pity for him, the master of that servant released him and forgave him the debt.
  The parable also comes with a stark warning:
  So also my heavenly Father will do to every one of you, if you do not forgive your brother from your heart.”