I didn't write this, but I did edit it. :-)

Unix

You shoot yourself in the foot.

DOS

You keep running up against the one-bullet barrier.

MS-Windows

The gun blows up in your hand.

Windows NT

The gun is so huge and unwieldy that you have to keep swapping it from one hand to the other.

OS/2

Point to body and click, point to leg and click, point to lower leg and click, point to foot and gun goes click

Mac Finder

It's easy to shoot yourself in the foot -- just point and shoot.

AIX

You can shoot yourself in the foot with either a .38 or a .45.

IRIX

The Terminator shoots you in the foot. A T-Rex bites your other foot.

SVR4

The gun isn't compatible with your foot.

Minix

You learn how to shoot yourself in the foot with a Saturday Night Special.

Linux

Generous programmers from around the world all join forces to help you shoot yourself in the foot for free.

HURD

You'll be able to shoot yourself in the foot Real Soon Now.

VM/CMS

IBM shoots you in the foot.

VMS

\FOOT\ ambiguous: supply more toes.

AMIGA-DOS

The gun works pretty well, except that few people use one and it's impossible to find bullets.

Mach

The bullets work pretty well, but they don't make guns for it any more.

Cray

You shoot yourself in the foot with an Uzi.

MasPar

You shoot all of your friends' feet simultaneously.