Not necessarily undo MS, but rather that the deal looks attractive but may actually not be any good. In other words, it could end up being one of those deals where a few years down the road MS has to write the purchase price off as a loss.
Businesses rarely (if ever) fail because of an acquisition gone wrong. But a acquisition gone right can have great returns.
If Instagram flopped when Facebook bought it, Facebook would still be doing just fine. But since it did go well.. yeah.
So I believe that it's still probably in the investor's best interest if MS does the deal even if it's only like a 35% chance that it isn't a complete failure.
Of course they mismanaged it and it should have been a reverse takeover like Next/Apple.