While testing the patch from #2325649, I discovered a need to work around iCalcreator.
iCalcreator knows that certain iCal properties (e.g. ATTENDEE) can have multiple entries in a single VEVENT. So when you access ATTENDEE for the first time, it gives you the first entry. Accessing it a second time gives back the second entry, or at least it tries to. If there is no second entry, iCalcreator returns False. Since Date iCal does not yet support multi-entry properties, I had to work around this functionality to ensure that hooks which add custom mappings don't run afoul of this process.