Before installing Arch on a USB flash drive, I disabled ext4 journaling in order to reduce disk reads and writes, being fully aware of the implications (file corruption after unexpected power loss). I was confident that I would never have to pull the plug or the drive without issuing a normal shutdown first. Unfortunately, there was one possibility I hadn't considered: sometimes, there's that one service preventing your PC from turning off, and at that stage there's no way to kill it (besides waiting for systemd to time out, but I was impatient).
So I pulled the plug. The system booted fine, but was missing some binaries. Unfortunately, I couldn't use pacman to restore them because some of the files it relied on were also destroyed.
This was not the last time I went through this. Luckily I've learned my lesson by now
Firefox as well: