If you do as Gil says, they may try to charge you a fee because you "lost" your original departure card.
Last time I flew out, I forgot to take the departure card to the airport so I filled in a substitute card, as Gil suggested. I then had to argue for 10 minutes with the immigration official who wanted Rp200.000 because the substitute card wasn't stamped with my arrival date. Eventually, I asked to see his supervisor and he begrudgingly backed off.
