When you accidentally merged two git branches of different repositories.

Every goddamn time.

Boss: Hey, how is the project going?
Me: promising. We have some basic functions working, but at this point it is more like a proof of concept.
Boss: Ah ok, I see.

Couple of days later...

Boss: I talked to a client who was very interested in the stuff you are just building. Made a really good deal! We need to be live by the end of the week.
Me: What?
Boss: What?

When you and your team invested a lot of their weekend time into the project to meet the clients unrealistic deadline and get not even a simple "thank you" from management...

Fax machines connected to VoIP connections...
Had a nightmare recently, where my fathers machine that he really needs refused to work after he moved appartements.
Uncounted calls with different tech departments, a furious fathers and two weeks later they found out, that they forgot to activate the protocol.