Considering that bug gave a guild (and anyone else who could replicate that bug) GM powers? Yeah, I’d say it was the most significant bug. GM powers are not supposed to be accessible to anyone but GMs.
And we don’t know how many people abused that bug. We only know that one guild found a way to get access to those powers and ripped through the Undermine raid with them, which put a lot of eyes on them for all the wrong reasons.