Ahhh.
Like the time I received a call from a customer telling me our software wouldn't talk to their database. The conversation went like this:
Me: Can you see if the database is running.
Customer: It seems to be.
Me: Hmm. OK. What processes can you see.
Customer: a couple of ksh processes.
Me: That's not your database. Can you try and start it.
Customer: I don't know how. I don't usually do this job.
Me: OK - cd into this directory....
Customer: I can't - it's not there.
Me: Huh? Where is it??
Customer: Well, we had run out of space so I deleted some files....
Yep - the customer had deleted the entire database. Not only that, they hadn't had a successful backup for several weeks!