"An internal build error has occurred. Right-click for more information"

This has to be the most irritating compiler error description ever in the Flex compiler in Flash Builder 4 for Eclipse.


I opened up the "error log" view in Eclipse, and saw that Macromedia (now Adobe)'s parser was throwing an exception "java.lang.ClassCastException: macromedia.asc.parser.MetaDataEvaluator$KeyValuePair cannot be cast to macromedia.asc.parser.MetaDataEvaluator$KeylessValue"


No help there.


I then found the issue. In my case, it was a bad coding that was actually valid syntax:

private var fileLoadIsComplete = Boolean = false;

This should be 

private var fileLoadIsComplete : Boolean = false;

My only advice is to start commenting out recent code until you find the offending line that caused the "internal build error".