This is something I just found out, and I figured I'd blog about it for those that might come across this in the future.
I'm writing some code in a project (JZBot specifically, http://jzbot.googlecode.com) that uses java.lang.reflect.Proxy. At one point, I was calling a method on a proxy instance, and it was throwing a NullPointerException. The weird thing about this was that the NullPointerException was being thrown from within the proxy instance itself, not from the InvocationHandler that it is supposed to call. Other methods on the proxy worked, indicating that there wasn't some error with the invocation handler somehow being set to null.
Eventually, I found the problem: I had declared the method on the interface that the proxy implemented to return a boolean, but the invocation handler was returning null. The auto-unboxing that was being done by the proxy instance internally was messing things up. I changed the method's return type to void (which is what it should have been), and everything worked.
This is yet another example of where to watch out for autoboxing and unboxing. Admittedly, it would be more helpful if the NPE had a message stating something like, for example, "null can't be unboxed to a boolean", which would most likely have saved me quite a bit of time. Oh well.