The difference is that after the fact--ie. once the program is implemented and you can try a scenario and see empirically what happens--what happens is no longer undefined.
Even if you can determine what's happening from a finite number of runs, all you can do is study the behaviour of existing implementations.

But the phrase the behaviour is undefined is much stronger than that. It says something about any other possible implementation; specifically, it says that you cannot determine the behaviour of a particular implementation by looking at other implementations.

And don't come with "but there's only one implementation of [Pp]erl". There isn't. 5.10.0 differs from 5.8.9. And 5.10.1 will differ from 5.10.0. Even 5.10.0 build with options X, Y, and Z on platform W will differ from 5.10.0 build with options A, B, C on platform D.

In reply to Re^4: The behavior is [sic] undefined by JavaFan
in thread The behavior is [sic] undefined by John M. Dlugosz

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