I was in Creative on Mac, just digging straight down, bored. Then I saw a glint of green. Emerald! The rarest ore in the game. This was the first time I had seen it spawn naturally. I spawned a diamond pickaxe and making sure I was holding the controls down properly, tried to mine it.
I forgot that you can't mine ores in Creative.
And so my precious emerald was lost, never to be seen again and to this day I still say "Why didn't I switch to Survival?!"