He ordered a beer.
He ordered 2 beers.
He ordered 0 beers.
He ordered 999,999,999,999 beers.
He ordered -1 beers.
He ordered 3.1415926535 beers.
He ordered “A String” beers.
He ordered Null beers.
…
I can’t remember where I first saw this joke. But it illustrates testing user-interface code…the rule is, the user might do ANYTHING and the code better be robust against it.