let the cat out of the bag

(idiom) to reveal a secret by mistake

Example Sentences:

Mom: How did our son know we bought him a bike for his birthday?!
Dad: I let the cat out of the bag when I told him by mistake.

Larry let the cat out of the bag when he arrived for the surprise party at the same time as the guest.

From the Blogs:

Car Auction Secret: Don’t let the cat out of the bag

Media:

Similar idioms:

1. cat got your tongue
2. give away
3. d-bag
4. spill the beans
5. like a cat on a hot tin roof
6. surprise, surprise
7. a doggie bag
8. check a bag