One of the "biggest" pitfalls, is probably simply choosing to ride with other riders that are going faster or slower than your pace. Often, double century rides become or seem much more difficult than they need to be. This usually happens when the cyclist starts a ride without a healthy respect for the type of conditions (weather) or type of route (hills) that the rider is going to endure.
Simply put, I figure the biggest pitfalls to completing and enjoying a double - is starting out too fast for yourself, the weather or the route......... Make a mistake about anyone of these items and it's no fun.......