“No space left on device” despite being less than 100% used

We were encountering issues writing to a mount point on a server:

[test-server] ls > dean
-bash: dean: No space left on device

The file system was 18% free:

[test-server] df -h
Filesystem            Size  Used Avail Use% Mounted on
/file-system           99G   77G   18G  82% /mount-point

The issue was caused by the inodes being used up:

[test-server] df -i /mount-point
Filesystem            Inodes   IUsed IFree IUse% Mounted on
/file-system         6553600 6553600     0  100% /mount-point

Working on the assumption that the largest directory would have the largest number of files

du -h /mount=point | grep '[0-9]G' | sort -nr -k1

I identified the mount point and cleaned up the files.