Ruling in tourney

[Duck]

Tournament:
Heads up on the river, first to act (player A) is contemplating an amount to bet, but opponent (player B) mistakenly thinks it's showdown time and flips over his cards.

I always thought that in a tourney you cannot expose your hand until the hand is over. If you do your hand is dead... But I wasn't fully confident.

If the situation described above was in a cash game would the ruling be different?