Not really the worst but it happened last week. Our lead Dev reviewed a pull request based on works that have been validated several times.
Suddenly, 5 minutes before leaving to take a plane for the week, he tells me that everything is to be refactored because it has no semantical sense, even if it worked perfectly and could be refactored later.
He told me that the day before the delivery and rewriting and restesting everything that had been done took 2 days, several out of worktime hours and huge stress phases. The delivery had to be delayed twice.

I'm easily stressed and a lot of things went by with bad timing (one other coder who could help me was not present, another had to leave early for his kids etc...)

That refactoring was unnecessary but it gave a bad impression to our client and made feel bad fr something I wasn't responsible for.