One of the guests decided to be a photographer for the day, and ruined a great deal of shots.
This probably happens more often than you think. I shot one wedding where the ceremony ended and NOBODY left the church because EVERYONE wanted to take pictures. I literally had people behind me bumping into me while I was taking pictures. That's the worse case scenario so far for me.
Now, what I'll do is I'll have the wedding director move the bridal party into a private room until everyone has left the church. This way after everyone has left I can go ahead and get the shots that the bride and groom want from me.