From The Jargon File (version 4.4.7, 29 Dec 2003) :
PEBKAC
/peb'kak/
[Abbrev., ?Problem Exists Between Keyboard And Chair?] Used by support
people, particularly at call centers and help desks. Not used with the
public. Denotes pilot error as the cause of the crash, especially stupid
errors that even a luser could figure out. Very derogatory. Usage: ?Did
you ever figure out why that guy couldn't print?? ?Yeah, he kept cancelling
the operation before it could finish. PEBKAC?. See also ID10T. Compare
pilot error, UBD.