Question : Java says "write once, run anywhere". What are some ways this isn't quite true?

Reponse : As long as all implementaions of java are certified by sun as 100% pure java this promise of "Write once, Run everywhere" will hold true. But as soon as various java core implemenations start digressing from each other, this won't be true anymore. A recent example of a questionable business tactic is the surreptitious behavior and interface modification of some of Java's core classes in their own implementation of Java. Programmers who do not recognize these undocumented changes can build their applications expecting them to run anywhere that Java can be found, only to discover that their code works only on Microsoft's own Virtual Machine, which is only available on Microsoft's own operating systems.