When using:
$(el).observe('keypress',fxHandler)
to have the handler check for the enter key and stop a form submit, everything works fine with Prototype 1.5.1.1 but is broken in Prototype 1.6.0_rc0. I have not been able to determine a patch as yet.
Very simple test pages illustrating the bug can be found at:
www.mile26.com/bug_prototype15.htm (works as expected)
www.mile26.com/bug_prototype16.htm (BUG: cannot stop event from submitting form)
I put severity = major as this bug breaks backwards compatibility and can cause unexpected behaviors. I also do not know if there are other cases where events bubble unexpectedly.