BOFH
Ha, there was a BOFH episode a few weeks back that was pretty much this, but with signs reversed --users complaining about a change that they think was made, but wasn't...
Thanks someone up there for the Hawthorne effect, I now know its name. Happens a lot in academia too; bioinformatics grad students complain program is not running, blah blah, so you go sit there right next to them... and it "magically" works now. "But I did exactly the same thing before!", they say. Uh-huh, I say. :-)