I'm posting this mainly for my own recollection down the road, because the old gray matter doesn't seem to retain as much as it used to... ;^)

While working in Mach-II today, I came across the following error:

A ResultArg/Key has been specified on a notify command method that is returning void. This can also happen if your listener method returns a Java null.

After checking through the code in my listener about a dozen times, I finally realized that I was getting this error because my function in my listener had a returntype of "void", but my "notify listener" tag in my event-handler (in the mach-ii.xml file) had a resultArg specified.

Oops! I took out the resultArg and all was well.