Do you mean leading a spade before spades are broken? Or playing your card before it is your turn?

For the first one, that is an illegal play. It is the same as any illegal play. The hand gets canceled and the score is updated in the most favorable way possible for the other team. You assume the offending team gets set, their nil(s) if any get set, and the other team makes their bid and nil(s).

For the second one, I believe that is basically an exposed card. If you show a card to the table, then it stays face-up on the table and must be played at the first legal opportunity. If you just played a bit early before your turn, then there isn't really any penalty. You just get to play it when your turn comes around.