Update (11:30 am PT): The problem seems to be resolved now, as we've successfully made a purchase. We'll keep an eye on it.
Original story: Steam's payment system appears to have crashed, with less than an hour to go before the Halloween sale ends. The problems were first reported on Reddit, after which I went to Steam and tried to buy a 59p platformer. The transaction hung for a while, before returning the line: "There seems to have been an error initializing or updating your transaction. Please wait a minute and try again or contact support for assistance. "
Obviously not great timing. We've contacted Valve for comment, and will update as we learn more. Guess even virtual bank accounts just get too full sometimes.