Speaking While Intoxicated

This one should be a new form of misdemeanor. If your wedding traditions include the bride or groom making a speech to the assembled guests, do it before you enjoy the free bar. You'll be thankful later when you watch the wedding videos, and you won't have to apologize to people you don't remember insulting.
Beware the Infamous "Wardrobe Malfunction"

Black underwear, white dress. It was a bad idea in elementary school, and it's still a bad idea today. Check in a mirror in various lighting situations to make sure that your dress isn't sheer or invisible in certain conditions prior to the ceremony. If you are renting a tux, check it for rips and make sure you have it fitted before you bring it home. Also, black is the only acceptable color for a tuxedo.
Late to the Altar

Not only is this one inconsiderate to the guests and your soon-to-be spouse, but it also wreaks havoc on your vendors. Ice sculptures melt, food gets cold, and photographers lose needed time to pose people and get those still shots for memory albums. The only reason you should ever be late to your own wedding would be something completely beyond your control, like a force of nature.
Running out of Food/Drinks

You made the guest list. You know how many people you expect to be there. Running out of food is a good way to make sure your guests feel unappreciated and is inexcusable. This is not the place to save money. Make sure you have enough dinner and drinks for all who are expected to attend; it's better to throw some away than have hungry guests and nothing to feed them.
Know Where the Ring Is Before Walking Down the Aisle

It happens more than you might think. The best man is supposed to be holding the rings, and he leaves them in his other coat. The bride expects them to be brought on a pillow but the ring-bearer is only eight and thought they were golden Barbie doll crowns. Do one more spot check for the rings before you start down the aisle and you won't have to worry about an awkward moment and a ruined ceremony.
Inviting Your "Ex" Without Telling Anyone

Unless there is an extremely good reason for inviting your ex-boyfriend or ex-wife, don't do it. It makes everyone uncomfortable. A close corollary to this is not letting members of the wedding party know what your betrothed's name is (yes, it sounds unbelievable but it does happen).

If you have not been engaged for a long time, or you recently ended a long-standing relationship
prior to your wedding and your wedding party isn't familiar with your intended, make sure you introduce them. You don't want to deal with a wedding toast to you and your ex-girlfriend because the best man didn't know your bride's
name!