When there's a subreddit about something you're interested in, but it's run by mods who enforce a extensive collection of esoteric posting rules.
We're sorry, but you've posted about Topic C on a Wednesday, which is strictly prohibited. Discussion of Topic C is only allowed in the megathread which is only open for comments on the first Saturday of odd numbered months. Didn't you read our rules?
also, you need 1 million karma to post and your account must be 100 years old. oh and you're shadowbanned in that sub, so nobody can see your posts, because a mod once read an unrelated comment you wrote in a different sub and didn't like it.