Attempting to resize the system partition without backing the system up first (right after I spent the whole day setting everything up). Gparted failed, the system did not boot any more so I had to stay up the night to redo the whole setup. No personal data loss since the system was fresh, though.
Since I got into Linux via virtual machines and Raspberry Pi before using it as my daily driver, I made most of my stupid beginner mistakes (like changing permissions on systen files) where it did not really matter.