No space left on device /dev/null

If it's impossible to copy a file with a size more than a particular volume then very likely the reason of that that /dev/null has been overwritten by ordinary file:

# cat 1GB > /dev/null
cat: write error: No space left on device

A solution is to recreate /dev/null as a character file:

> rm -f /dev/null (get rid of the file I had put there)
> mknod /dev/null c 1 3 (some unix magic: char device, major #, minor #)
> chmod a+w /dev/null (permissions)

It works.