While testing a web application on the Steam developer site, Artem Moskowsky came across a bug that yielded strange results.

Moskowsky told website The Register, that by altering a couple of parameters, he was able to generate 36,000 Portal 2 keys.

Although the codes were the the kind that could activate the full game on Steam, they were old keys which had been generated before and were therefore likely to have been used already.

Moskowsky reported the problem to Valve who awarded the developer a handsome sum of £15,000.