/dev/full - Wikipedia
In Linux, FreeBSD, and NetBSD, /dev/full, or the always-full device,[1][2] is a special file that always returns the error code ENOSPC (meaning "No space left on device") on writing, and provides any number of zero bytes to a process that reads from it (similar to /dev/zero).[3] This device is usually used when testing the behavior of a program when it encounters a "disk full" error.
$ echo "Hello, World" > /dev/full bash: echo: write error: No space left on device