What are possible complications of diverticulitis?

Abscess / Stricture. Abscess formation in the abdomen is a possible complication. This is a collection of bacteria and stool within the belly. Long standing diverticulitis tends to cause strictures of the colon that can lead to acute bowel obstructions.
Abscess, Fistula... Acute diverticulitis develops when a diverticulum on the colon ruptures. Usually the body seals this hole closed with only minimal inflammation. On occasion, an abscess develops that requires drainage. In some cases, the hole is sealed by the wall of an adjacent organ such as the bladder or vagina. The inflammation can create a connection between these structures, called a fistula. See comment.