Took me a while to figure this out and it doesn't seem to have been mentioned, so if it can save anybody else time...
If you have child drop list with a value field and display field that is filtered by a parent drop list value, when you change the parent value the child drop list will display the first entry of the new filtered list but the value field won't change. Not good; the user thinks they have made a change but they haven't. It will only change when the user selects a different entry. This is because the child value field is only primed if the session value for that a field doesn't already exist. Solution: delete the SessionValue of the child value field whenever the parent value is changed.
Topic: Priming a child drop list when a parent drop list changes (Read 3765 times)