Problems, changes will happen. Life cannot be planned, even though we try our best to avoid disruption.
The weather is unpredictable, if your servce is outside and it rains, can the venue bring you indoors?
Important guests my have travel problems, do you wait for their arrival or proceed on time.
What happens if the Officiant is late or doesn't show up at all, is there a back up?
Gowns get lost, cakes get dropped, photographers forget film, univited guests show up, parents get in arguments. The list goes on.
It is impossible to have answers for every scenerio that may arise, just remember, a solution will be found and your day will proceed, and it will be the best day of your life, up to that point.