Sometimes there are really strange problems which leave you mystified as you can't find a sensible cause for the problem at hand.
The follow problem had me stumped for several minutes:
One of our webhosting customers tried to install phpNuke on his account.
He did it by the book, extracted the files, edited the config.php-file, configured the right database, everything seemed to be perfect.
Curiously, accessing his phpNuke pages with a webbrowser only resulted in the message "No input file specified".
As he could't find out why, he asked our support.