My first ever software developer job, I was hired with basically no knowledge or experience to learn (I was very lucky). I knew MS-DOS command line pretty well from my childhood, but hadn't ever used POSIX. I was given a macbook air and some docs to follow.

Trying to follow the docs, supplementing with a lot of googling, I somehow managed to remove the tar program from my system. This broke literally everything. Had to stop halfway through the multi-day process to do a clean reset and start over from scratch.