Rewind said pretty much everything already.
The awful Auto-shot, Multi-shot, Cleave (& some others) bugs were never present during beta (or maybe in 2010 you know), and MC was run countless times without the link glitch reported last week or any other (well, maybe not all, haven't checked myself, just read the forums) being detected, so some kind of "regression" definitely affected the server in the mean time, maybe because they "ported" some old scripts to a new core like Rewind said, or new fixes weren't handled properly or whatever, I have no idea.
It is probably why they decided to develop their "test automation" system, which hopefully will prevent that in the future.