[lmm]

>I suppose you've never had a problem with two JVMs that ran a package but didn't behave quite the same way, to disastrous effect?

Only at the very limited level of "versions of the JVM before x have a bug that makes this package not work.

>Or dealt with software that only worked with one very specific version of the JRE that was exclusive from the equally specific JRE required by another bit of software?

No, never. I know one should never overestimate enterprise software vendors, but you'd have to try really hard to make something that crap.