It is always hard to envisage disaster, but it can happen. As best man, your responsibility to the groom is to ensure that the stag party is one to be remembered. Should something unforeseen happen, however, you could well be left with a stag party that is a complete wash out, unless you have a contingency plan.
Transportation problems: Flights / trains cancelled or a car breakdown can ruin a stag party before it’s begun. There is not much you can do about the actual problem and, depending on the severity, will make all the difference if you wait for the next flight or train or abandon the whole trip. If it is the latter option, try to have an idea of where you can go and what you can do, as you have to try to salvage the stag party somehow.
Injuries: Drunken guys can always become injured and should one of the party end up in hospital then the stag party can be considered over. Prevention is better than cure in this case so try to keep the gang fairly sensible without sounding like their dad.
Arrested: We all know it can happen and again prevention is better than cure. So if one of the group seems to be losing control, steer him away from the people who might take offence.
This is but a few examples of possible awkward situations that you may find yourself in. If you have a contingency plan, however, or are aware of potential problem areas before you go, you will find that you can rescue a stag party, which could have otherwise been a disaster.