Form behaves differently in Acrobat versions 8 & 9

Hi,

I just had an interesting issue - a dynamic xml form (saved as a PDF) behaved differently in Acrobat versions 8 and 9.

In Acrobat 8 it worked fine.

In Acronbat 9, it displays popup messages for mandatory field warnings every time it is rendered e.g. "field cannot be left blank"

The fix?

What I did was create a new blank form, transfer or copy the entire content of the old form to it. Magically the error went away.

But the question is - what would have caused the form to behave like this? What is the difference between Acrobat versions?