FWIW, the club are the ones that may be in trouble, not the interchange official.
As a trainer, I have to let the IC official know we are going to make a change, the player is given a card by us with the number of his interchange. The player walks down the sideline with the IC official and when the player coming off is seen by the IC to be off the pitch he will take the card from the ongoing player and hold it up to the crowd.
I cannot understand how a player got on unless the club's sideline staff did not inform the IC official.
In the NRL, you can acutally be fined if you throw the card at the IC official and also, you MUST put it in their hand. I've seen players called back off the pitch to be told to picked up a dropped card and place it in their hand.