When someone hears obvious telltale signs of one things, and assumes it's something similar but off the wall. Origins from the show Scrubs
Guy 1: I had a Zebra Diagnosis the other day.
Guy 2: What happened?
Guy 1:I heard galloping and whinnying so I assumed it was a zebra.
Guy 1: Man, I don't feel so good.
Guy 2: What's wrong?
Guy 1: My nose is running and I'm tired, I bet I have swine flu!
Guy 2: Dude, whatever. That's a zebra diagnosis, you have a cold.