Steam accidentally cooked up something truly frightening for the end of its Halloween sale. At some point today the store's payment system appears to have crashed, preventing users from making purchases around the time the sale was scheduled to end. Sorry to you last-minute shoppers. The ability to purchase games through their store seems to currently have mixed results.
Based on panicked posts and inevitable memes, it looks like the trouble started a couple hours ago at around 5pm GMT. Down Detector shows a similar timeline of user reports flowing in. As for right now, success seems mixed.
Alice0 helpfully attempted a purchase and was given the message "There seems to have been an error initializing or updating your transaction. Please wait a minute and try again or contact support for assistance." I, on the other hand, am now the proud new owner of $0.99 game Otherside.
The Halloween Sale has ended now, unfortunately, but you can still try your luck with getting a purchase through on everything else.
I've reached out to Valve to see if they've identified the cause for this truly spooky development (whether it was Halloween-sale related or not) and will update to let you lot know what they say.