I was using sdk version 4.0 and everything was fine.
I installed 4.1 and started getting this runtime error almost immediately after running:
VerifyError: Error #1053: Illegal override of UIMovieClip in mx.flash.UIMovieClip.
at mx.managers::SystemManager/deferredNextFrame()[E:\dev\4.x\frameworks\projects\framework\s rc\mx\managers\SystemManager.as:267]
at mx.managers::SystemManager/preloader_preloaderDocFrameReadyHandler()[E:\dev\4.x\framework s\projects\framework\src\mx\managers\SystemManager.as:2460]
at mx.preloaders::Preloader/timerHandler()[E:\dev\4.x\frameworks\projects\framework\src\mx\p reloaders\Preloader.as:488]
I'm using a few .swc's that contain flex components that were made using the Convert Symbol To Flex Component command in Flash CS5, and disabling the instances of those components in the Flex project stops the error from occurring.
I read here ( http://forums.adobe.com/thread/779472 ) that this might have something to do with the version of the flex component kit I use.
I am using the Component Kit for use with Flex 4 SDK projects from http://www.adobe.com/cfusion/entitlement/index.cfm?e=flex_skins however I noticed that when I remove the extension in extension manager, the Convert Symbol To Flex Component and similar commands are still available
All SWCs have to be based on 4.1. You can't mix versions.