so, a trap i've always wanted to spring on a party of adventurers is to have them primed for unexpected undead to attack from out of dark narrow corridors...

then have them face what appears to be a flying, glittering skeleton that must surely be some mega powerful skeletal lich like critter, only to have them eventually discover it's just a normal mundane skeleton stuck in the transparent form of a gelatinous cube.

they can waste all their anti-undead magic and abilities on a big blob of jelly.