http://blog.plover.com/2017/04/02/
"A Unix system administrator of my acquaintance once got curious about what people were putting into /dev/null. I think he also may have had some notion that it would contain secrets or other interesting material that people wanted thrown away. Both of these ideas are stupid, but what he did next was even more stupid: he decided to replace /dev/null with a plain file so that he could examine its contents.

The root filesystem quickly filled up and the admin had to be called back from dinner to fix it. But he found that he couldn't fix it: to create a Unix device file you use the mknod command, and its arguments are the major and minor device numbers of the device to create. Our friend didn't remember the correct minor device number. The ls -l command will tell you the numbers of a device file but he had removed /dev/null so he couldn't use that.

Having no other system of the same type with an intact device file to check, he was forced to restore /dev/null from the tape backups."