Long-time rper, I'll vouch for 'dead OOC' - yeah, sometimes they can be all in the IRC, but if you're in the rp and the OOC's dead, then either they're not in the IRC, or they are and haven't told you. The latter can be a symptom of poor communication that, later, will result in people getting mad at you because your character actions interfere with a plot developed in IRC that everyone knew about except you.
Related, if one player starts plotting around their character and someone else's character without notifying the other person. That's just asking for a grudge to develop.

Also vouching for the GM who won't take criticism even for a blatant logical fallacy. Like, let's say the rp's backstory would influence player's characters' backstories and personalities in a very significant way. However, the GM does not share this backstory until about halfway through the rp, and he's sharing it because he's complaining about how his 'shades of grey' are being considered purely evil and unreasonable. This being directly the result of the fact that he did not inform players of relevant information at the rp's beginning. Also related, a GM who cannot be flexible. I'm sure their plot was very interesting and epic and all, but sometimes you just have to either get VERY creative in getting the players back on track, or cede the fact that they want something else and roll with it. Insisting OOCly that they need to follow the story the GM planned in defiance of characterization, logic, and common sense should be considered a red flag and a sign to leave as fast as possible. 

And, one last bit for a sign to avoid - a GM placing restrictions on player characters and then proceeding to violate those restrictions. I don't mean something like 'your characters wouldn't reasonably know about the legendary artifact we're looking for but mine does because they gathered you all here'. I mean stuff like 'no one can have an edgy past' and then writes one for their character. 

I know there's a lot of focus on GMs. I'll probably post later for warning signs from players.