Usually, when I am staying at a hotel, I return the access cards to the front desk when I leave.  I have no use for the cards and I know that they can be reused.  But, during my recent stay, not only did the hotel reuse the access cards, they used “white out” on the card key packet to change the room number.

Of course, if they really wanted to save money, they would just reuse the same packet for the same room and not waste the “white out”.