While doing some research on it, I did find that there is one possible hazard: explosions.
If you consume an indigestible sugar alcohol such as sorbitol (found in cough syrup and sugar-free gum), it will turn into hydrogen and methane gas in your digestive tract. If you then pump in air (21% oxygen), you have a fire hazard. Along with a camera, colonoscopy devices often include an electrically-heated wire loop. If your doctor spots a polyp with the camera, he may decide to burn it off with the wire. If you drank a lot of cough syrup the day before, your doctor's office will need a major cleanup.
Fatal colonic explosion during colonoscopic polypectomy.
Bigard MA, Gaucher P, Lassalle C.
A patient is described who sustained the first reported colonic explosion during colonoscopic polypectomy. Mannitol solution was used for bowel preparation, and the colon was completely clean. During snare removal of a cecal polyp using high-frequency current a loud explosion occurred. In spite of emergency surgery with transfusion of 45 units of blood, uncontrollable hemorrhage persisted from multiple bleeding points, and the patient died.