aceofhearts,
I have waited for someone else to respond to your question, but no one has.....so, I'll give it a shot. First of all, there are many situations that should be clarified with more detail. Any player showing their hand with action pending violates poker etiquette and is highly unethical. In a tournament, a warning and possibly a penalty should be enforced. I will give you one example that falls under the situation you mentioned, but might not be considered unethical. Example; Final betting round, Player A checks, Player B checks, Player C checks and Player D turns his cards over for all to see. If I were dealing that hand, I would consider that action a check and I would read his hand and wait for the other players to respond by folding, or turning over a better hand. I think "head to head" action might be looked at differently than a player turning over his hand with a full table of players (highly unethical). In a cash game, it would be looked at as a foolish move by many but, as long as a player is in the hand and has not released his hand into the muck, it is "live." I'm sure there are a number of situations that we could cover that are more severe but, I think we have enough to get something started.