When a site doesn't redirect you to where you were after it nags you to log in to see content
God I hate this! Never fails, either. I'd be on a page of someone's profile or some piece of content I'd like to see but I'm required to log in. Okay, I will do so. So I do and it doesn't redirect me back! It just puts me in the front page, like no thanks, just bring me to where I was!
Went to hackernews the other day. Tried to upvote a comment, then it asked me to login. Put in my login, hit the button, and boom, it redirected back to the original page, and the comment I was attempting to upvote is successfully upvoted! I was expecting I'll need to press the upvote button again, but I don't even need to do that. My mind was blown. Wish more websites do this.
It's a great predictor of how the rest of your interaction with a site will go. If a dev doesn't have the space and motivation to give a shit about details like this, they'll cut corners on other shit too. It doesn't always mean they're bad developers; more often they're just rushed.