This is just pure downside. I throw it in only as a reference point, not trying to tell you what to do or not do. Just consider this as 'worst case' and go from there.
* The wedding photos are all most people have to remind them of a very special time in their lives. Fail to come through and you have left them with a very long-lasting, ugly, bitter, memory. Friendships sometimes do not weather such storms.
* The courts are filled with lawsuits against wedding photographers who did not deliver the goods, or who delivered less than the customer expected. Seriously:
http://news.google.com/archivesearch?pz=1&ned=us&hl=en&q=wedding+photographer+sued&cf=all
* Cameras and flashes and memory cards and film can all fail. Backups are mandatory. Two of everything AT THE MINIMUM. If shooting digital, you MUST have a backup device ON SITE to make frequent backups of your shots. Having a card fail is not acceptable, period. If your card fails and you did not back it up on a wedding shoot, you are the stupidest person in the entire world and deserve to get your house sued out from under you, which is what is going to happen.
* It does not matter what your best friend, closest relative, etc, says to you when they're begging you to shoot their wedding about it being OK if it turns out badly. It won't be OK, and there WILL be hard feelings. Promise.
* Liability insurance is a must. Let some guest trip over your ladder or tripod and break a hip and find out. It does not matter that you're just doing it for funsies. The lawsuit will not be for funsies.
* Second shooter required. For a number of reasons, not least of which is for redundancy of the main photographer. Cars break down, people run out of gas, get the date wrong, get lost, etc. You cannot fail to do as you've agreed to do - there are no do-overs for wedding photography.
* You must have a shot list.
* You must have permission BEFOREHAND from the venue and/or minister. Some allow flash, some don't, some allow external strobe and lights, some don't, finding out on the day of the wedding is very bad.
* Go to the rehearsal.
* Stay for the reception.
* You are not there to eat, drink, socialize, or dance. Bring water and a sandwich and stay hungry or you will miss something.
Your day will be miserable, with little or no food or water, few breaks, constant battery / film changes, constant consultation with shot lists, keeping track of guests leaving early or arriving late whom the bride (who has turned into a harpy from hell at this point) wanted shots of together, in-laws who don't like each other and won't stand for group shots, formerly-married parents and grandparents of bride and groom who show up at the wrong time or start fights with the new husband/wife, etc. Guests who get in your way, get drunk and become obnoxious, demand to see your camera kit, comment how they could/would/have done it better, and lots of people with point-n-shoot digicams blocking your shots.