Since Apple refuses to make their bug database public, I’m posting a summary of how to recreate a (very) annoying bug in iCal (at least on Leopard, 10.5.2):

In iCal: When creating an event in the day/week/month overview, and than dragging this event to the to do items and setting an alarm (by just choosing “Message”, which defaults to showing a reminder 15 minutes prior the to do items due timestamp), that alarm’s timestamp isn’t respected (when re-visiting the to do item, the item’s alarm is set to “the day before, at 23.45″).

This bug has been filed at Cupertino as rdar://5867919.