It was not a bug per se. Drunken Choco explained it here - they had two different branches/builds to test. One was with bug fixes, the other one was with balance changes.

However it was noticed late in development that both branches made changes to the same part of code and can not be merged together, as one branch will overwrite the changes from the other. Even worse, apparently they had conflicting changes as well. To simplify it, both builds were working fine but it was not possible to patch them together within the game code.

So they needed to rewrite the parts of the code that clashed between the two builds.