Upon merging the recent branch of my current Flex/AIR project to the main line, I received the following runtime error:

VerifyError: Error #1053: Illegal override of FlexModuleFactory in mx.core.FlexModuleFactory.

I went back and double checked the compilation settings for the project, the Flex SDK, the AIR SDK, third-party libs that are being used. All the same. Clean building the project didn’t help either. Not to waste time I put some debug points in my code and realized the origination of the error to the Flex 3 compiled CSS/SWF that the code was trying to load.

StyleManager.loadStyleDeclarations Loading a stylesheet compiled with Flex 3 (probably via Flex Builder) throws this error. Simply, recompile your CSS/SWF again, and you should be good to go.