Around me, it's common to help people through your list. There are a couple of our tournament grinders who will politely nudge you out of their traps if they're doing a more casual testing game: you move a model that's in threat range of one of his more convoluted combos "I mean.... I'm not sure I'd make that move right now" and if you ask, he explains what he plans if you do commit to it.
As for people using gotcha-moments as part of the core gameplay, rarely intentionally. We all have a lot of rules on the table, I don't blame someone if they forget something once or twice, I've unfortunately done it to someone a few times myself. It helps that most of us either aren't playing super seriously, and the folks who do are chill enough to undo some things for forgotten rules.
I have encountered a couple folks before, usually much younger folks who aren't 100% keyed into the social contract part of the hobby, and/or folks who are more in the mindset that the game is fun if you win, with a bigger win being bigger fun. I'd like to add that it's at a person-to-person level, I do not believe that all people in either/both of these groups do this, or that it's only people belonging to either/both of these groups, these are just the trends I've seen.
The one that gets me is when 2 separate units of the exact same warscroll are deployed together (to serve as a slightly-harder-to-wipe screen or whatever), move together (to speed up the movement phase), but have no way to mark where 1 unit starts and 1 unit stops. I've had multiple games where I'll charge in, do 30 damage to this block of 10, and be told "no it's 2x5, remember" when no, I don't, because I forgot about it since they're all grey plastic with nothing to show the end of 1 unit. I should remember it, yes, but it annoys me that I have to remember the position and unit of individual models in your army.