I installed the updated Java Source Files module (version 
1.7.1.1.1/200112041930-jdk14), which "has parser and internal compiler based 
on JDK 1.4".  However, I'm running NetBeans under JDK 1.3.1_01 (so perhaps I 
shouldn't have updated it, but it's too late now).

Now, every time I open a file in the Source Code editor, the icon in the 
Explorer gets a red cross (tooltip "Errors during parsing." and trying to 
expand it just shows a single "Error" entry.  When I close the file in the 
editor window, the explorer entry goes back to normal, and can be expanded to 
view the class etc. just fine.  It's only while they're open in the source 
editor that there's a problem, and it does it with every .java file.