Before playing any tournament, always check the specific local rules. There is still no standard international set of poker regulations; string bets, out-of-turn plays and verbal declarations are defined according to the whim of each tournament director, and you don't want to be surprised.
In one heat of the knockout TV tournament, Premier League Poker, I moved all-in from the button with no hand. My cards were so bad I don't remember them - 64, 85, something like that. Tony G showed me K9 in the big blind. I shrugged. He put his cards back down on the glass and started thinking. "Hang on," I thought. "He's shown me his cards - surely the hand is dead." But I wasn't sure. The tournament director, Marty Wilson (one of my favourite people in poker), said nothing. Had he not seen what happened? Or, who knew, maybe the rules of this specific tournament stated that cards could be shown during action, to increase needle and horseplay.

I didn't want to shop my weak hand even further by calling for a ruling, without being sure it would go in my favour. Tony G eventually made the call and knocked me out; I subsequently learned that of course his hand should have been dead. I can't blame Tony, whose actions weren't challenged. I couldn't blame Marty, who can't see everything. It was my own fault for not being 100% certain of that tournament's rules. So: always investigate fully before you sit down. And decide in advance: either you will always call for a ruling in confused circumstances, good hand or bad, or you never will. That way, the choice you make can't give anything away.