One shot cards

[Hatcher123]

So there are a few cards that are pretty powerful that don't say discard once used. Like mate, help me out, or out to lunch. Are these still allowed to be kept by the player after use? That seems too powerful.

[NightShiftLoser]

Mate is essentially a monster card, so once it's defeated, you would put it in the discard like any monster.

Out To Lunch plays the same way as a Monster Enhancer. It would read like "(Insert Monster) is out to lunch" the same as it would say "Humongous (Insert Monster)". You would discard it with the monster.

As for Help Me Out, I'm pretty sure the logistics of it, is that you need to discard it for it to take effect.

[MunchkinMan]

According to the base rules, the only cards that remain in play after having been played are Classes, Races, Items (including one-shot Items that aren't being used right then), and Treasure cards that provide permanent bonuses (specifically in the original Munchkin, Hireling and Really Impressive Title). Any other card gets discarded as soon as its effect is complete (e.g., one-shot Items that give a combat bonus and Monster Enhancers are discarded once the combat is over). Obviously, there are going to be exceptions (usually worded so on the cards, or in new rules that come with an expansion), but none of these are an exception. So, all of those cards you listed would be discarded as soon as their effects were resolved: Mate, when the combat was over (it's a Monster); Out To Lunch isn't a Monster Enhancer, as suggested, but it does completely end the combat and gets discarded with all the other cards played during that combat.