There are many medical reasons that can cause black stools that are tarry with a foul smell, but sometimes dark stools are caused by eating certain foods or taking iron supplements. However, black stools can also be a result of bleeding, and the darker color indicates that the blood could be coming from a point in the upper gastrointestinal tract.
Gertrude Levandowski survived a 96-hour procedure at a Chicago hospital to remove a giant ovarian cyst; the longest surgery. The second longest surgery ever was 40 hours long and that was to remove 6 limbs from a 8 limed baby in India. But the most common longest surgery is spinal cord surgery it is a 14 hour surgery.
The longest operations I've been involved in have been living donor liver transplants and intestinal and multivisceral transplants. While an uneventful operation with favorable anatomy may take only 8-10 hours, a complex one may take up to 18 hours. In most cases there is a break in between. With two teams working simultaneously (donor and recipient teams), it is common for one team to take a break and wait for the other team to catch up.