The moment when you accidentally delete the final product instead of the experimental one because they have the same prefix and the shell's completion choose the final product when you type the name.

That happened to me today. I accidentally deleted a postfix calculator that I wrote in Scala instead of the sbt one (Which does nothing) because both of them have the same prefix (nimtha is the program's directory name, and nimtha-sbt is the sbt one). I don't notice that until I go back to the project directory and don't see the program's directory. I tried to recover it with TestDisk, but it can't. All because of fish's shell completion, and also because of me.

At least that was a pretty small project so I don't feel very bad.