I screwed up permissions on an LXC container in Proxmox by converting it from unprivileged to privileged (against recommendations) and had to mount it offline and write a script with find into chown via the execute flag to change all the UIDs and GIDs from the shifted unprivileged ones to the standard host-level ones.

Luckily this was in my own lab so it was a (mostly) harmless learning experience.