// Yes, this is the wrong way to build an XML file.
// But you know what? It works, and we still have time
// to go out for drinks after this is built.

Source: Direct link to the source code
Software: Opentape
Programming language: PHP

# I was thinking… If they invented comments, why not use
# them? So there you go, here’s a comment. And you know what?
# I actually like comments. Yeah, that’s it. Much better,
# fills in some empty spaces like this. What could have been
# here without these comments. ah?
#
# Shit, I’ve nothing left to say… Ah, well. See ya in the
# next version. :)

Source: Direct link to the source code
Software: File::Mode
Programming language: Perl

/* This is a gross hack to solve a problem in linux kernels
 * 2.1.85 and above. Please, children, do not try this at
 * home, and if you ever see anything like it, please
 * inform the Gross Hack Police immediately
 */

Source: Direct link to the source code
Software: Adaptec Aic7xxx driver
Programming language: C

/*
 * God only knows what the original author
 * wanted this undocumented feature to
 * do.
 *   %5++ is really r7
 */

Source: Direct link to the source code
Programming language: C

// Happy scrappy hero pup FIFO queue. Or a List. But that’s
// just, like, my opinion, man.
[...]
// By the way I’m on drugs right now, so this code might not
// make too much sense or work.

Source: Direct link to the source code
Programming language: C#