Some of the more common mistakes I can think of can be either technical or not. All things considered, forgetting that you can tell people what to do in order to get the shot can be a big detriment. If you're not shooting for a close friend / family member, it's not like you're trying to make sure that everyone likes you. You just want the bride and groom to be happy with the final result, so if you have to tell Bob over there to put down his beer and smile, you've got to remember that is ok.
It sounds simple, but sometimes it's harder than it seems.
Then there are some simple technical problems like forgetting to charge a battery or not having a spare, or not having enough memory cards or having one break. But those are easily solved with good planning.
And then there are the more technical mistakes, like shooting with the wrong white balance (Raw is good here), or forgetting to change the ISO for the conditions.
Uh, hope that doesn't put you off
lol I know that a lot can go wrong, so it's knowing that and planning how to fix things that go wrong that can make all the difference
