monkey trap

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun literally, probably folk-lore A cage containing a banana with a hole large enough for a monkey's hand to fit in, but not large enough for a monkey's fist (clutching a banana) to come out. Used to "catch" monkeys that lack the intellect to let go of the banana and run away.
• noun figuratively A clever trap of any sort, that owes its success to the ineptitude or gullibility of the victim.