How to ruin someone's life forever? Just tell them about something they will constantly encounter but can't fix.
Example: one day I was enlightened that there is no such thing as two differences; there is only one difference, and therefore "that's two big differences" is an incorrect expression.
Now you live with this too 😈
Drop other stories in the comments. звОнит and экспрессо don’t count :)
P.S. By the way, how do you say 'killjoy' in English? I need it for a tag.