One stubborn (but not very good) dev working on one part of new project (Windows desktop application with C# underneath) decided he didn’t like the interfaces we were agreeing for the algorithmic code.

Instead of discussing with the team (we were still very much in design phase), he made his own interfaces with the same name but in a different namespace, and in his assembly rather than in the base library. He was senior to the rest of the dev team, so when we raised our concerns he pulled rank and just carried on.

I resigned not long after that.