a piece of cake you cant eat
A piece of cake you can't eat is when someone brags about how easy something is, and it turns out to be the worst thing ever.
You said this job was easy. I was working overtime. You said you'd help. You were on vacation.
He said the math was simple. I cried in the bathroom. He said he'd pass. He failed.
She said the cake was good. It was inedible and had a hair in it.