[mindcrash]

Changing code you have committed working on (as can be seen in the product management tool) without notifying you, which makes git return hundreds of merge conflicts when merging the main branch into your work branch -- basically ensuring you either can start over again based on the new code or risking drama when force rewriting the main tree without said unexpected changes.

Worst time it happened to me was the day just before the start of my Christmas holiday and it almost made me quit.