I sent a 1.6-only binary to a QA team with a 1.5 JVM. I decided that the best way to solve it was to move my development environment to 1.5, and just use 1.6 to run applets and ordinary programs. I was already using -source 1.5 in my ant file, but I had forgotten to tell IDEA. When I downgraded, I recompiled, and found a very amusing compiler bug in the latest release of Java 5.