I know I personally can't stand to see characters act out of character for the sake of drama, but I recognize that not everyone behaves consistently, either, and sometimes a little stretch for the sake of the story may be necessary (and I'm aware that some people will disagree with that statement). The titular question crossed my mind when I realized how much better the conflict would be if my MC's mentor simply didn't tell her about his plan. After further thought, I decided his decision isn't that bad ("protect her from tremendous guilt and a possible existential crisis" vs "be honest out of respect"), but it still got me thinking. How OOC is too OOC?
It's too OOC if you can't 1) come up with a internally satisfying reason for the character to do what they're doing and 2) find a way to show the reader what this reason is based on previous actions/thoughts. Sometimes we run into problems with #1 when we love the idea of a piece of dialogue or a scene, even though it doesn't really mesh with the character. Been there, done that, so I totally understand; but if you're truly writing something for a character that serves you as the writer but not the characterization, it's time to trot out that old chestnut and kill a few darlings. #2 is really important. I've definitely had to go back and work examples/thoughts in retroactively to show why my character would do something seemingly OOC, after feedback from a beta reader/editor. It's hard sometimes to remember that readers don't live inside our heads and know what we know, and it's only with outside assistance I can see where I've messed up on that front.
For me, the line is crossed when it genuinely doesn't make sense within the narrative. You are 100% correct that we - humans - are anything but perfectly predictable and we do indeed engage in utterly unintuitive behaviors all the time - each and every one of us. But a story is not real life, in the same way that the dialogue found within any book is not remotely representative of our normal modes of day-to-day discourse. So, I'm down for a character making a strange decision so long as that strange decision has some kind of rationale within the character's envelope. If it's just because "plot works better this way", and I feel the shoe-horn, then that's the crossed line.