The disk is full and nothing is using it

The application stopped taking orders: it writes them into a temporary directory, and large writes fail with "No space left on device". The space keeps shrinking although nobody puts anything there. The on-call engineer checked how much was used, then checked what was using it, and the numbers did not agree.

The symptom

The /tmp filesystem is nearly full. Walking the directory turns up a few kilobytes.

$ df -h /tmp
Filesystem      Size  Used Avail Use% Mounted on
tmpfs            64M   61M  4.0M  94% /tmp

$ sudo du -sh /tmp
4.0K	/tmp

Both commands are telling the truth. They simply count different things, and that is the whole scenario.

What you have to end up with

Three conditions. /tmp has at least 40