(Yeah, instead of using another person's question, i decided to make a seperate thread.)
My thought for an example/question would be calendar entries in something like Google Calendar.
Is there a natural key here? My thought says no, unless you force one on (userid,startdate,enddate,entryname).... but is that efficient? Does it violate the convention of avoiding business logic in keys (since theoretically, someone could create the same event twice)?