Oh, I just remembered another one or three. So, resizing the partitions. My install at the time had a swap partition that I didn’t need anymore. Should be simple, right? Remove the partition and the corresponding fstab entry, resize root, profit. Well, the superblock disagreed. Fortunately, I was lucky enough to be able to re-create the scheme as it was, and then take my time to read the wiki and do the procedure properly (e2fsck, resize2fs and all that stuff).
Some people I’ve met since, unfortunately, weren’t so lucky (as far as I remember, both tried to shrink and were past mkfs already) and had to reinstall. The moral is, one does simply mess with superblocks; read the wiki first!