"Helpful" door cards - one shot cards?

[Aar]

hello, I've returned to Munchkin (classic version) after many years to play with the kids, and I have a question. When I throw a curse at someone, it is obvious that the card is dumped afterwards. But when I use a helpful card like "out to lunch", illusion etc. It is not stated the card should discarded after use but this is how I understand it. Am I wrong?

[Brf]

All of the cards that are played into a combat are discarded when the combat is over.