I tracked down a very frustrating error today that turned out to be a non-issue. The problem was in Eclipse with a new project I was configuring for one of our developers. The project is supposed to produce 1.3 compatible classfiles and Eclipse was giving me the following error:
'Incompatible class files version in required binaries' followed by the path to the rt.jar file in my JDK directory.
I couldn't see what was different about this developer's system from all the others that were similarly configured without error. I finally figured out that this error has a setting under Preferences... -> Java | Compiler | Building -> "Incompatible required binaries". The developer having trouble had this setting set to "Error". On my system, it was set to "Ignore", which apparently is not an issue.
I suppose we might run into trouble in the future if we try to use the jars produced in a 1.3 JVM, but I kind of doubt it.