Personal story: I once remember seeing a spider in my childhood, as huge as an elephant, with its web spanning two large trees. I am sure I saw it, without a doubt. But my conscience says that it's simply impossible.

I kept on telling my dad this story when I was a kid and he didn't believe it. But when I told it a long time later on, he too thinks he might have seen it.

Maybe he formed those memories from my story.