This one is a doozy. I hope it gets fixed by a hot fix rather than the next "dot" update. Bjorn Jensen gets credit for finding it. I ran into it myself yesterday when working on my blog entry. If you have a CFC that returns numeric data and uses a returnType of numeric, then ColdFusion will throw an error. Here is one example of code that will throw the error:

<cffunction name="getX" access="remote" returntype="numeric"> <cfargument name="id" type="any"> <cfreturn> </cffunction>

I then entered this URL to test: http://localhost/test.cfc?method=getx&id=1&returnformat=json. The error was:

java.lang.Double cannot be cast to java.lang.String

The fix is to simply change the returnType to string (or any). You can find more detail (and vote for a fix!) at the public bug tracker: