How unethical is this?

Different people have different answers. I tell people once if I see their cards, sample size 3.

Depends on the game I guess. If its serious people need to take it serious. If its fun/recreational then just say something. I would probably even say something at a casino but that's just me. Plenty of people would use it as an angle and say nothing.

I tell them twice and after that it’s on them. If they don’t react well the first time I tell them then once it is.

If we’re heads up like in this scenario I would tell them I saw their cards before the flop comes out and offer to check it down. Unless I don’t like them, in which case I’d bet big and tell them to fold.

nah bro you’re fine. if he’s showing his cards to half the table like a billboard, that’s on him. you didn’t angle, you didn’t lean, you just saw what he leaked. using that info is literally what poker is about

I also have a tell-them-twice rule of thumb.

If I know I'm folding, I'll fold and tell them quietly they're exposing their hole cards. If I know I'm calling or raising, I'll tell them loud enough for the dealer and others to hear. The second time I tell them, I'll tell them firmly that they need to protect their hole cards.

If I see a player somewhere else at the table lifting their hole cards off the table to look at them, or otherwise being careless, I'll point it out to the dealer. If it keeps happening, I'll ask the floor to get involved.

The only time I can remember someone next to me doing it repeatedly even after I tried to get them to stop, it was a really old man on my direct right who was doing a ton of limp-folding, such that me seeing his cards didn't change anything. I would have raised the same hands and folded to all his raises regardless.

Ultimately, it is the player's obligation to protect their own cards. I think it is generally a nice thing to do to let a player know that you saw their cards, but if you are actively involved in a hand, play the hand. There is no reason for you to lose a hand just because you saw your opponent's cards. I mean say that you had a set and they had a higher set. Are you going to call their all in? Just because that's what you would do if you didn't see their cards? Of course not.

Win the hand, let them know they are flashing their cards. I only tell people once. If you can't figure it out after that it's your problem. Now if you're involved in a three-way pot, I think ethically it's more complicated, because then you have an unfair advantage over the third person in the pot. And in that case, I would feel obligated to tell the other person in the pot what cards were exposed.

Use the information. You can clean up like Postle.

This is one of the reasons why I prefer to sit at the end of the table rather than in the middle.

Tell him once or twice, then use the info to your advantage if he doesn't start protecting his hand. I would have played the hand the way I normally would have played the hand, but that's me.

Been in this spot a few times at the casino ... say something preflop before you act, or just fold, is by far the easiest response. Would guess that at least half of the times I've thought "Eh, I don't feel like speaking and I only know a bit of information" it's turned out to be significant (like here).

Would also be more likely to speak up the more bad/obvious the flash was. Mainly for two reasons: 1) The bigger the mistake, the more likely a mistake will happen again. 2) The bigger the mistake, the more likely other people saw it.

You can say different things depending on how sure you are, and I would now not say "I know" unless I'd be fine betting my entire net worth on it (a few times I've thought I "knew" V had 3d but he actually had 7h or similar misidentifications).

If I didn't say anything before I acted, very likely to never say anything.

I would probably warn him "protect your cards dood, I'm pretty sure I saw them" and go from there.

GcluelesspureevilnoobG

This isn't really a strategy question, and there have been a zillion threads on it in in the casino and cardroom poker forum, so I'm going to just lock it instead of moving it there. Cliffs: different people have different answers.