I was an intern at a university security lab working on a 7 months project. Early on, I figured it would be a good idea to use SVN to save my work so I setup a repository and did a few commits but quickly stopped maintaining the repo.

One hour before the end of my internship, I was ready to leave, my work done, ready to be used for the next person taking over the project. I want cleanup my files and documentation so it is all tidy and I try to commit my work. Of course SVN cannot commit because the repo and my work have nothing left in common. So I type (on a Linux system): svn delete to cleanup the repo so that I can push my files... I lost months of work and I was not able to recover my lost files from the file system... I had to leave for my country of origin since this internship was part of an exchange program. I felt so bad about it, it still haunts me.