Apache's "File-Extension-Feature"
Many years ago, someone mentioned on a congress that apache has an interesting feature: if apache doesn't know a file-extension, it will just take the next one. If someone saves a file called "shell.php.ab", apache would not know what to do with the extension ".ab". So it will just skip this one and uses the next one and the file "evil.php.ab" becomes "evil.php" and gets executed.
