Christi touched on character on Monday. I'm going to expand on that a bit, because those TSTL moments are horrendous, but in some cases they are less annoying than simple bad decisions. They're the same you say? Not necessarily.
A bad decision can be anything from taking the wrong job to making a wrong turn that lands you in the middle of a bad part of town. With a flat tire. Bad decisions happen. In books, though, there are things that I call Bad-Bad Decisions - decisions that don't make sense for the character to make. No, I'm not talking about the stiletto-wear-idiot who goes into a dark attic after hearing a noise when she knows a killer is on the loose. That is a TSTL moment.
I'm talking about the character who is a buttoned-up-brain who, in the middle of a book, decides to get a tattoo in the middle of his forehead. For no reason. All of a sudden that suit is shucking the shirt and tie for biker wear and a tattoo. That makes me throw the book at the wall. I finished a book very recently where one of the main characters, a memo-writing, calendar-obsessive, everything-has-its-place suit had a one night stand. No reason. No motivating factors. Just had a one-nighter..which, of course, put the two characters on a road of discovery. BUT, for the rest of the book, the character was lost to me. He never addressed why he acted out of character; it was never explained and I couldn't reconcile that one decision, it didn't make sense. I wanted to like the character, but that one bad-bad, out of character, for-no-reason decision made me nuts.
Now, do one nighters happen in real life? Yes. Do strict, have to have a list people sometimes act out of character like that? Yes. But they have reasons for doing the things they do. Just as our characters need to have reasons. For me, the action for the sake of action decisions are bad-bad..they take me out of the story and it's only rarely that I ever recapture the magic of that story.