Forget the rain, what about...
WIND! Ice cold gusty wind!
This is what happened on my wedding day. Sunny as can be in the hills east of Santa Barbara, CA and the craziest winds that lasted for days!
We luckily could move the reception inside into a farmhouse, but a ton of money was wasted on decor of the outside space : (
Also I'm in talks with a lawyer right now about having to pay for damaged patio heaters! Three fell over the night before the wedding due to these crazy winds! During the wedding they didn't even work - with the exception of 2 - and the ones that did work didn't really matter b/c the wind just blew the heat straight sideways!
I still had a beautiful and memorable day, and everyone said that they dug the intimate atmosphere of the farmhouse (and the warmth it provided!)... but ladies beware of your rental companies and what they will make you responsible for, even if it is Mother Nature at work! : )