RangeError: Error #2006: The supplied index is out of bounds.

I sometimes get this error during runtime and I have no idea what cause it. The mouse disappears on the window. Anybody?

RangeError: Error #2006: The supplied index is out of bounds.
at flash.display::DisplayObjectContainer/addChildAt()
at mx.managers::WindowedSystemManager/http://www.adobe.com/2006/flex/mx/internal::rawChildren_addChildAt()[E :\dev\3.0.x\frameworks\projects\airframework\src\mx\managers\WindowedS ystemManager.as:1568
at mx.managers::WindowedSystemChildrenList/addChild()[E:\dev\3.0.x\frame works\projects\airframework\src\mx\managers\WindowedSystemChildrenList .as:111]
at mx.managers::CursorManagerImpl/showCurrentCursor()[E:\dev\3.0.x\frame works\projects\framework\src\mx\managers\CursorManagerImpl.as:491]
at mx.managers::CursorManagerImpl/setCursor()[E:\dev\3.0.x\frameworks\pr ojects\framework\src\mx\managers\CursorManagerImpl.as:351]
at mx.containers::DividedBox/http://www.adobe.com/2006/flex/mx/internal::changeCursor()[E:\dev\3.0. x\frameworks\projects\framework\src\mx\containers\DividedBox.as:881
at mx.containers.dividedBoxClasses::BoxDivider/mouseOverHandler()[E:\dev \3.0.x\frameworks\projects\framework\src\mx\containers\dividedBoxClass es\BoxDivider.as:312]