Internal fistula is MOST commonly found between gallbladder and:

Biliary fistula is a complication of gallstone (90% of biliary fistulas are due to gallstone). The inflamed gallbladder becomes adherent to adjacent stomach, duodenum, colon and necrosis develops at the site leading to the development of fistulas. Most common site of fistula is the duodenum. Colon is the 2nd most common siteof fistula. Ref: Hamilton Bailey 10/e, Page 432.