I once got a call-out on Boxing Day to "fix" an installation of some Christmas software on a Windows machine.
After a few unsuccessful attempts, I removed the CD and wiped a fat, greasy thumb print off it (probably turkey-flavoured). Everything then worked fine.
"Twenty-five quid??? For wiping off a thumb print!!!", he shrieked.
"No. Twenty-five quid for knowing how to", I answered.