In my case, the easiest way to halt me while I'm reading, is the characters behaving needlessly disrespectful.
Kids in school behaving not just as wiseasses, but pretty much insulting the teacher and not even giving them a chance for example.
I can't begin to count the number of fics where the writer thinks she's making the characters behave awesome by having them talk back to the teacher, especially when there's no real reason for it. And then expects us to side with the little snot that's doing so.
Now sure, in that case we're dealing with teenagers, so it can be completely in character for them to behave like brats. (hell, I'd be surprised and shocked if Dean suddenly started acting like a well behaved model student. But that still doesn't stop the fact that if he started calling his teacher names, that he'd get detention and that John would tan his hide over calling undue attention to the family. And if you expect me to believe that Sam, Sam of all people, would be a jerkish little brat who thinks he's better than his teachers, without a seriously damn good reason, provocation and so on, then you instantly get me to stop reading.)
But worse than that is cases where adult characters behave like that.
Whether it's an oc being the kind of jerk that customers_suck like to go on an on about (and believe me, I've worked in the service sector long enough to know when there's damn good reason to start complaining about certain customers) Or worse a newbie special agent treating a senior special agent like their nitwits without a clue and telling them to their face that they're stupid and have nothing to say over said newbie, and yet despite such utterly unprofessional behavior and being a complete idiot, they don't get fired or you know, sent off to some desk job after a specifically bad review or you know at least get racked over the coals for their horrendous behavior. In fact what annoys me most about this kind of behavior that the chars in question tend to get treated as if they're utterly awesome for doing so.
Maybe it's just me, but you do NOT insult your boss to their face, without some serious provocation, and even then it's questionable. It's stupid, childish and not something you tend to get rewarded for in real life.
And no, in most cases, people are not going to side with the newbie against the established co-worker, especially if said newbie has done nothing whatsoever to prove themselves.
So, does anyone else have a pet peeve like that?