If you tell about something that was supposed to be a secret, we say you let the cat out of the bag.

VOA: special.2009.09.27

"My sister has been in renal failure for months now." "You're supposed to give her a kidney?"

VOA: special.2009.06.12

It's a memory from childhood or it's eating something that you know you weren't supposed to, but you did it anyway and really enjoyed it.

VOA: special.2010.02.17

(SPEAKING) It's supposed to be the ultimate home viewing experience, designed to put you in the game or give you the thrill of 3D in a big theater.

VOA: standard.2010.03.11

"It depends on what you are talking about. If you are talking about some money that bleeds around the edges to get a road paved, but you get the road paved, you get it paved on time, you get it paved to the quality it is supposed to be, frankly it is a nuisance,"

VOA: standard.2009.11.17