When you receive an apparently free, wonderful gift with high and/or unexpected maintenance costs not previously considered by, or obvious to, the recipient.
Mary: Grandpa is giving little Elizabeth a pony for her 12th birthday if she promises to take care of it! Isn't that great?
Bob: What?!?! How are we supposed to pay for the board, food, vet bills, and riding lessons for Elizabeth? Free Pony? Give me a break!
Bill: So I meet this hot chick, Sandy, who takes me on a 7-day cruise at no cost because she's filthy rich. Then two weeks later I've got crabs and herpes. What a free pony she turned out to be.
Jim: I can't believe I won that new Corvette in the church raffle! But since I'm unemployed and living with my mom, I can't even afford the gas and insurance, let alone maintenance.
John: Oh, man! Sounds like you won a free pony! Well maybe you can sell it and still get something for your trouble.