With so many puns in Munchkin, it would be astonishing if there wasn't one you didn't get now and then. This troper's younger sister one day just spontaneously realised the pun in "Fire Arms". This troper's version involved picking up the "Lust Monster" card and then suddenly remembering the D&D "Rust Monster". — Count Dorku
The Sushi Knife of Doom in Munchkin Cthulhu. At first you wonder why a sushi knife gets to be so doomy. Then you realize that nearly all sushi is composed of seafood, and that many of Lovecraft's monsters were reminiscent of sea creatures (he had a deep fear of the sea) and that in Lovecraft Lite and Flanderized works (like, I don't know, Munchkin Cthulhu), eldritch abominations are nearly alwaystentacle monsters.
This troper didn't realize the pun in the females-only weapon "Broad Sword" for two days after first using it.
Absolutely best is the strongest monster of all, Plutonium Dragon. Which is, of course parody of the Platinum Dragon (Bahamut) in AD&D.