A tweet from 2019 where someone did a time travel campaign, but accidentally added a mountain that wasn't there in the present. The players were wondering what happened to the mountain, and the DM was wondering that too. Eventually, they ran a poll, and decided it got up and walked away.

This community, back on reddit, went absolutely nuts over that. There were a LOT of memes.

I saw a post a dm talks how he forgot a mountain that exist in past, and how players exited to see what happened to mountain.