Is Magic: The Gathering a consistent game?
The game can never end up in an undefined state.

There are rules for every situation, and they don't contradict each other. It is possible for the game to go into an infinite loop because of card/rule interactions, but the players have to either break the loop by doing something else, or the game ends in a draw, depending on the exact circumstances. In competitive settings such as a tournament (from world championships down to local prerelease events), there are also official judges present who can resolve any rules dispute.