Okay, I haven't told this story for a long time, and it's Christmas, so here we go:
When I was dating my first wife, I went to her parents for Thanksgiving dinner. Among the dishes on the table was blackberry jello with grapes in it. Seemed like a 50s kind of dish, but whatever. I took some of everything, and planned to clean my plate. My future MIL was telling a story when I put the first bite of the jello in my mouth, and my brain screamed that something was horribly wrong. I thought there must have been something rancid in the jello or the grapes - the grapes didn't even have the right texture. I was about to spit it out - it was revolting - when I realized it was a taste I'd had before, not something rancid. All this was really just a moment, but it seemed like forever before it clicked: it wasn't grapes, it was green olives. She made blackberry jello was green olives in it.
I thought for a moment that it was a prank, though that family wasn't the pranking type, because no one else had taken any except the mom, but she had a mound of it and was eating it. I finally said, "It was surprising to bite into a grape and find out that it's an olive," and everyone tittered. Future MIL said that no one else likes it, but she does, so she makes it for herself.
It should have been a warning.