Unplanned events and disasters are inevitable, but they still add another "surprise" to your party.  Whether guests see it as a good surprise or bad surprise is how you handle it.  A handy fire extinguisher can turn a potential disaster into a comical mishap.

Plan for it so you will be ready when it happens. 

Alcohol

Buy more liquor than you think you will need or keep the phone number for a liquor store that will deliver handy.  A cocktail party with no cocktails is over.

Upset or Injured Guest 

Immediately isolate the problem.  If the guest is upset about anything less than the kitchen engulfed in flames, take them to another room. Simply say, "Let's go in here and talk about it."  

If a guest is injured, treat them in another room, no matter how minor.  If bleeding, wrap and cover the injury enough to prevent a trail of blood through your house while moving the guest.

Ideally, keep a First Aid kit on hand.  You may encounter a cut from a broken glass or splinters if your party is on a wood deck.

Food disasters 

First, keep guests out of the kitchen unless they are helping you at that moment.  Not only will this keep your guests safe from exploding blenders and splattering oil, it will minimize the impact of any missteps in the kitchen.

Fire can be reduced from a major disaster to a minor incident by having a charged fire extinguisher handy.  If you don't have one, buy one for the most likely places a fire could start (kitchen, garage, party area).  If you do have one, make sure it is up-to-date and fully charged.

Do not store fire extinguishers above the stove.  In the event of a fire on the stove, it would be difficult to access.

Smoke can be a nuisance. Once you eliminate the source of the smoke, disconnect your smoke alarms before they go off.  Open nearby windows and use a fan to blow air out of the room.

Inedible hors d'oeuvres is probably the most common cocktail party disaster. Keep a backup emergency hors d'oeuvre on hand, should you need one.  Obviously, something that you can put together in a jiffy.

It is also a good idea if your backup hors d'oeuvre does not require electricity to prepare, in the event of a power outage.

For example, I keep 2 or 3 bags of peeled & deveined cooked shrimp in my freezer for emergencies and impromptu parties.  Don't forget the cocktail sauce.

Alternatively, prepare an extra batch of hors d'oeuvres, in case things don't go as planned or extra guests show up.  You can always serve the extra batch at the end of the evening or freeze them for your next soiree.

If you have the opportunity, do a practice run of each appetizer.  Prepare all sauces and anything that can be frozen before the party, to minimize mistakes before serving.