Idiom: to let the cat out of the bag – to reveal or tell secret information by mistake, to reveal facts that were hidden before, to let a secret be known without intending to

Context #1 

Three Friends are talking about a surprise party next Friday

Dan: Hey, Juan! Did you hear about that surprise party on Friday night? It’s supposed to be really big! Everyone is going.
Juan: Really? No, I haven’t heard about it at all.
Debbie: Come here, Dan! You just let the cat out of the bag! How could you?
Dan: What do you mean?
Debbie: It’s a surprise party for Juan’s birthday. He doesn’t know about it because it’s supposed to be a surprise!
Dan: Oh, I see. Sorry about that. I guess he knows now.

Context #2 
Two Co-Workers are talking during a break
Sami: Hi, Petra. Congratulations on the promotion! I’m really happy for you.
Petra: What? Promotion? I have no idea what you are talking about.
Sami: Umm…our manager hasn’t talked to you yet?
Petra: No. I haven’t seen him today.
Sami: Well, I think I let the cat out of the bag. You got a really big promotion because of all the great work you have been doing recently. Everyone knows about it.
Petra: Ha, ha! Everyone, but me! Well, that’s OK. At least it was something good!

Meaning: To let the cat out of the bag means to tell someone a secret without intending to.  In Context 1, they are planning a surprise party for Juan and Dan accidentally tells Juan about his party. In Context 2, Sami tells Petra about her promotion before she even hears it from her boss.

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