As an example the RRULE passed in is as follows:
Which means the last tuesday of every month, if we did not have the BYSETPOS param this RRule would mean every Tuesday.
If you pass this RRule into the GetRecurrenceDateTimeCollection method on either the IRecurrenceBuilder or the ScheduleControl it will never return a value, it appears to get stuck.
However if you pass in the following RRule:
(As you can see the only difference being that we now have a positive value for BYSETPOS)
It will instantly return the DateTime collection.