I came across this amusing paragraph in the Diablo II entry on Wikipedia:
The “Secret Cow Level” is the result of a running joke from the original Diablo that spawned from an Internet rumor about the cows that appear in the game, seemingly without purpose. Supposedly, if the cow was clicked a certain number of times, a portal to a secret level would open. The rumor turned out to be a hoax, but the legend was born, and player after player asked Blizzard about how to access the level.
Long ago I worked for Blizzard Entertainment doing QA, and the product I spent the most time testing was the Mac port of Diablo. At one point I found a bug where the terrain tiles would be missing, leaving a pattern of white lines on a black background where the tiles should have been. I’ve long since forgotten exactly how I made it happen, but as I recall it was easy to reproduce and did involve a portal. By the time I found this bug the PC version of Diablo had already been out for months and the Internet rumors about a “secret cow level” were a source of much amusement (because we knew there wasn’t one).
When I found this bug, the black and white pattern reminded me a bit of a cow’s fur, so I wrote the bug up with the description “secret cow level” as a nod to the rumor. Of course it was fixed before the game shipped, and I’m pretty sure that only four people ever saw it (including me). So there you have it–the true story of Mac Diablo’s secret cow level.