Follow up:
So my wife's phone started acting up again.
1) Events which she added on her phone were not showing up in the Yahoo! calendar.
2) Changes made to Yahoo! calendar were not showing up on her iphone
So I cleared Safari cookies & history.
I also re-entered the account password as suggested.
Now:
1) New events created on phone show up on Yahoo calendar
2) New events added on Yahoo calendar show up on iphones
however:
3) Changes made to existing event on Yahoo calendar are not showing on her iphone
4) Changes made to existing events on iphone are not showing in Yahoo calendar
5) Events added or changed on the iphone (before clearing cache and re-entering the password) are not syncing, which means that a lot of events created in that period where there was no communicate are only on her phone.
Needless to say this is insanely frustrating when we're trying to coordinate multiple calendars across multiple devices. We basically have come to the point where we don't know which calendar is correct.
It's insane that there's no force refresh button on the calendar events page (not the list of calendars).
FIX IT Yahoo! or Apple.
Someone just tell us what the ##*#&&#& we're supposed to do.