Aha! Thank you Mathew.
When I switch the course format to "weeks", my HotPot module behaves exactly as you describe. Sorry for my earlier misunderstanding.
Strange thing is I have no recollection whatsoever of writing that code. It seems to have been added to the HotPot module in Moodle 1.6, 1.7 and 1.8. Very strange. I don't think we need it, so I will remove it.
Moodle 1.9 and later are not affected because they generate the forms in a different way - using the "moodleform" object.
The updated scripts for Moodle 1.6, 1.7 and 1.8 should be available from the Moodle downloads page and CVS soon. "Soon" used to mean "within 24 hours", but I think there is a different system now whereby changes to Moodle core code, of which the HotPot module constitutes a small part, have to be approved at a Tuesday meeting, so "soon" might mean "next week".
If you are in a hurry you may be able download the latest HotPot module for your Moodle from my server.