This is not a coding error and it is the same as how it has always operated in the current registration system.
True Dungeon is particularly prone to this because it allows up to 10 tickets per account - it's a trade-off to make it as easy as possible for a group of friends to grab a bunch of tickets and sort out exactly who is going later, but it also makes it easier for folks to accidentally double-book themselves.
It is important to always double-check your cart before actually checking out (especially since the wish list is set up to potentially allow you to book yourself to repeat events) - even more so if you are coordinating with friends to make sure that everyone has tickets for everyone else on their separate wish lists.
There is a 2-hour window to review tickets before checking out, so for groups of friends that are coordinating their wish lists that closely, they should also build in some time to confirm everything before checking out.
Gen Con LLC
Yeah, that's what I eventually figured out. We've done True Dungeon for many years, but we've always have 6 or more people playing, so we never had this double-booking problem. So, if one person nabs the 6 tickets, there are only 4 left, so the next person won't be able to get 6 tickets.
THIS YEAR, unfortunately, we only have 4 people playing TD, so we had 2 people each grabbing 4 tickets for the event.
To compound the problem, we saw that there was a Return ticket option, so we tried that, and the money was put in system credit. I contacted Customer Service about a refund, and was told that once it hits System Credit, it's in a black hole where no light or dollars can escape, and it can't be refunded. :(
(And of course, this didn't happen with a $6 RPG ticket, but has to happen with a $78* event.)
All of this together leads to a perfect storm where I have a monstrous credit card bill looming, and enough system credit to last me until 2020.
This has been a very expensive way to learn that (despite attending Gen Con for over a decade), I didn't really understand how event registration works.
* equivalent to 5 Platinum, 4 Gold, and 6 Copper pieces, for you D&D players out there