What are the possible complications of a colonoscopy?

Veery rarely. Bleeding and injury to the bowel. In experienced hands the combined risk is less than 1 %. You may have gas and bloating afterward but these should resolve quickly and you can resume normal activity and diet the next day. The risk of having an undiagnosed colon disease or cancer is greater than the risk of the procedure.
Several. Possible complications include: perforation, bleeding either from a tear or from a polypectomy, infection, a flare-up of diverticulitis or colitis, splenic injury from the scope pulling on the splenic ligament, gas cramps (side-effect not a true complication), flare-up of hemorrhoids.