Some other moments that I think that no one was expecting for:
Ross returning from China with a new girlfriend (season 1)
Ross sleeping with the xerox girl (season 3)
Ross hooking up with Janice (season 5)
Rachel getting pregnant (season 7)
Joey falling in love with Rachel (season 8 )
Janice appearing in the hospital in labor, the same day Rachel was (season 8 )
Mike coming to Barbados to ask Phoebe to marry him (season 9).
I think that Rachel and Ross getting married is indubitably the most unexpected moment.
What do you think? Let me know