Wedding mistakes are usually the result of poor planning or miscommunication with the vendor/waitstaff. Food can come out at the wrong time during the reception, the best man has no idea when to give his speech, the flower girl is too little and cries halfway down the aisle, the the cake went to the wrong banquet hall, the officient said the wrong names, etc. Other problems have to do with the guests, most of which involve alcohol. For example, a disgruntled family member who does not like the groom gives a drunken, offensive speech, while an intoxicated bridesmaid ends up crying on the dance floor.
These are just some of the many things that can do wrong at a wedding. Unfortunately, there is no foolproof way to avoid mistakes or disasters, even if you get married at City Hall or elope to Vegas. The best you can do is to start planning six months to a year ahead of time to find reputable vendors who take their jobs very, very seriously. It also helps to use discretion when making the guest list, such as not inviting an old co-worker who got blasted at every single Christmas party. And remember, even if disaster does strike, you are still marrying the person you love, so roll with it and have fun!