Wow. The magnitude of this blunder indicates some s-e-r-i-o-u-s organizational problems in one or both of these companies.
This isn't a simple oversight, or letting some "B" bugs slip thru, it means the patch was never Formally QA Tested. Which means:
No One -anywhere- in the development chain of command - from Grunt Coder I, to Project Lead, to Senior Producer, to Director of Programming...
No One ever raised the question "was the patch tested?"