to let the cat out of the bag (to accidentally reveal a secret)

Eg 1:

A: I’ve done something terrible!

B: What have you done?

A: I told  Brian about the surprise birthday party we are going to have for him!

B: Oh no! You’ve really let the cat out of the bag, haven’t you!

Eg 2:

A: Greg doesn’t know that you failed your driving test, does he?

B: Yes, he does. Helen let the cat out of the bag last night. She told him when she was drunk.

A: You can’t trust her with secrets. You shouldn’t have told her.