An Internal Build Error has occurred...

public var interactionType:String=INTERACT_DRAG=INTERACT_FORM;

This line of code just wasted 4 hours of my life.

Flex didn't notice that there was a problem, and it was only in building that a problem arose with the line 'Classes must not be nested.' and then the more obscure 'An internal build error has occured' Location: UNKNOWN.

So it was down to me looking through my thousands of lines of code to find the problem. I ended up restoring a backup of my code and comparing the two until i narrowed it down to the above line...