Is it normal to mix up smells?

I've noticed this for at least two years... Sometimes meat cooking downstairs smells like chocolate cake. Sweat often smells like greasy burgers to me. These are two that I remember off the top of my head.

Other than distorted smells, I occasionally smell things that aren't there, like plastic.
I googled it and read someone else mention that popcorn smells like pee to them, which I've noticed once, but I think it was just bad popcorn.

It's probably not normal, but how common is it?