Ok, just kidding. Should not be a new year's resolution. But serious, it was causing me nightmares over the holiday season. All of a sudden, several clients starting to have problem using an ActiveX control in IE (to be precise, ChartFx 6/7 ActiveX controls). After some really boring days researching, installing/uninstalling/reinstalling and testing, I finally found out .NET 4.5.1 was causing the problem.
I have no idea why it's a problem, but I know if I uninstall 4.5.1 and rollback to 4.5.0, then all the problems are gone. So, it's official, .NET 4.5.1 update is banned.