But they still don't think of all common user possibilities. I like this joke:

A software tester walks into a bar.

Runs into a bar.

Crawls into a bar.

Dances into a bar.

Flies into a bar.

Jumps into a bar.

And orders:

a beer.

2 beers.

0 beers.

99999999 beers.

a lizard in a beer glass.

-1 beer.

"qwertyuiop" beers.

Testing complete.

A real customer walks into the bar and asks where the bathroom is.

The bar goes up in flames.