1) Coach your registration desk volunteers on what to do or who to call if someone shows up without a ticket -- it might be a major donor.
2) Coach your emcee on how how to pronounce the names of people who will be recognized or thanked. Stumbling over the keynote speaker's last name, for example, won't endear the speaker to your group, and your guests may conclude that he or she is not so important after all.
3) Attend to the bodily basics -- make sure key people know where the extra toilet paper is, and that whoever has the master restroom key doesn't go home early!
4) Put a time limit on speeches and enforce it equally for everyone.
5) If drinks are to be served at a bar, choose a server who knows how and when to cut people off.
6) If you'll be thanking people, don't rely on memory. Bring a list that you've checked five times beforehand, and follow that with a blanket statement about the "many other wonderful people who have helped out in so many ways."
7) Get the food served on time. Hungry people get grumpy. If you're not having the event professionally catered, make sure that some of the people involved have served food to crowds before, and have a well-considered plan for getting everyone fed by a certain time. (Hint: Buffet lines are fastest for serving, with people able to line up on either side.)
8) Don't give anyone food poisoning. Check with your local health department for its standards on refrigeration and how long different items can safely sit out.
I'm glad to say that not all of these mini-disasters have happened to me -- but I've seen many of them happen! And most can be averted with some advance planning.