When they are tweaking the back end these sort of bugs will happen, instead if they do those and have extensive test before applying in the main application these sort of issues will be sorted.
It could very well be that they already did that, but the circumstances during the tests didn't trigger the bug. In that case it's very unfortunate. Another possibility is that they just assumed it was ok and then rushed to push it live, which if so, is a clear indication of incompetent developers and operators.
In the end, I think that when you're trading on a centralized platform, you after the risks of seeing an exchange get hacked and whatnot, also have to factor in these events. Good thing for the traders is that bugs of this severety don't usually happen. An exchange will always try to prevent compensating traders where possible, especially with how shareholders don't want their margins to suffer due to negligence.