These two items appear on every sync, but they alternate which direction they sync. In Other words MPE keeps syncing them, first in one direction and then in the other.
I think the problem was actually with GCalDaemon. I sync Lightning with Google Calendar using GCalDaemon, and then I sync the phone with Lightning using MPE. I updated the event's end date in Google Calendar, then forced the sync from Google to Lightning to the phone, and now the problem has gone away.
I tried that before, but I had problems due to the size of my calendar. The sync was very slow and the Thunderbird process grew to the point where my whole system became sluggish.