Here's my shot.

Trivial: when my friends taught me poker, they made up fake rules. After that, I tried explaining the game to other people, they just stared at me, stifling their laughter. Eventually I caught on to the joke!

Serious: way back when, I was eight or nine, one afternoon I was left alone with my beloved puppy. And I wanted to treat him, so I shared with him my chocolate bars. Alas poor Billy did not survive the night. Seeing how heartbroken I was, my parents had the vet lie, he told me the puppy had hurt himself with broken glass. Years later, I would learn I had poisoned him.