Avocado halves filled with mayonnaise
I was very young, in a townhouse in London with my parents and their friends, and they were at a long table all eating avocado halves, the stone hollow filled with mayonnaise and sprinkled with paprika. I got asked if I wanted a portion, said yes, and got given a little red yo-yo, but no avocado. I spent years thinking that "portion" was a word for a yo-yo or other small toy. I also spent years thinking that this was a waking memory, but my folks can't think of an event where that could have happened, so I'm now pretty confident it was a dream.
They did, however, start eating avocados like that after I told them about it. It's not very good.