A snitcheruse - You think you're the only one who knows the secret, but the

You think you're the only one who knows the secret, but the secret already got out and it was your fault.

I thought I was the only one who knew the principal was a fake, but my friend already told him.

I tried to keep the secret about the teacher's dead parrot, but my classmate spilled it first.

I tried to hide the fact that I ate the cake, but my mom already knew.