Wait until you run out of inodes!

Old war story: I had an old Sun 4/260 with 2 1G drives - I had SunOS on 1 and Gentoo on the other - my initial Gentoo install worked for a while but then the portage directory used all the configured iNodes - really weird errors and I could not figure it out at the time; error msgs maybe should mention inodes? I had to do #gentoo-sun IRC and someone suggested df -i which was indeed the issues (solve: you can configure extN filesystems to have more iNodes)

That happened to me exactly once in my 20-year career. It was on a web server (maybe even NGINX) that had too many cached files.

Even though it only happened once, I still set up monitoring for inode exhaustion.