I encountered yesterday a terrible issue in ActionScript 3 (Flex): files (with new File) suddenly won't open, and if you examine (watch) the File object you'd see a weird "function Function {}" string appended to file name.
The issue was traced to an extension of the Array class (using Array.prototype.myfunction), simply its presence will cause this bug.
(and yes, I'm doing Flex at the moment, having to translate some C# code to run in Air).