Memorize this word: Recto-vesical fistula.
Hopefully it won't be a word you become personally familiar with. Without getting too graphic, a recto-vesical fistula is a fistulous tract between a patient's bladder and rectum that can occur due to carcinomas, Crohn's Disease, irradiation, or trauma to the affected area.
Once again, a recto-vesical fistula is a fistulous tract between the rectum and the bladder.
There are many more details to this condition, but I think it's best that I leave it to those interested to do their own research. I would post a picture, but it's kind of nasty. Use your imagination.