Most programmers might have experienced the vexation after losting hundreds of lines of code, without saving to files. The causes can vary from unexpected OS crash to hardware failure, or even a naughty cat.
This morning I just found Wikipedia, likewise, also placed such a placard on its website:
Power corrupts. Power failure corrupts absolutely.
We’re currently recovering servers from a power failure in our colocation facility. This means backing up 170gb of database on several servers and running recovery. Back soon…
Wikipedia uses InnoDB. A sudden power failure often corrupts some data files and sets the database state into unrecoverable. Sure they have backup and redo log files. But it still takes time.