A friend just rolled back all changes made on the server during one month. BY MISTAKE.

He was about to roll back one day in a worldedit cuboid... but... he typed in the wrong month number. And he used rollback instead of werollback command.

Rolling back the whole server a month took a few minutes...
The undo command did not work. It did hang. After 3h of nothing happening we restarted the server.
So... one month of work is lost. Not so nice.

This is why backups exist

Agreed :)
But still. That mistake I described is easy to make.

He could have /he preview ....and like SmokeFlow said, get a backup plugin. I use a plugin which backs up all the worlds hourly and keeps 24 of them. I then backup the 'Backups' folder used by the plugin daily using Windows backup. I made the same mistake your friend did in the past, the backup saved my ass.