Check out this thread. http://boards.straightdope.com/sdmb/showthread.php?t=237108
Vomiting actual fecal matter is very uncommon, and generally due to an abnormal communication between the lower intestines and the stomach.
You need to provide a cite for saying that bacteria are added “along the way.”
Although bacteria do reach the colon from things we put into our mouths, the body does not, to my knowledge, ever add any bacteria anywhere.
The “flora” we have in our colons (mostly bacteria, but perhaps a few other beasties as well) don’t really extract nutrients either. That’s all done higher up in the intestines. (There are a few tiny exceptional cases in which the bacteria can be useful, but they are just not a major factor.)
Feces is composed of indigestible parts of food (mostly fibers, but a few other things as well) along with dead bacteria. This course outline page gives a breakdown:
Feces
1. Composition
a. water: 75%
b. solid: 25%
1. roughage (30%) of solids
a. insoluble fiber (e.g. cellulose, lignin)
- undigestible
- acts as a bowel irritant
- draws H2O out into lumen
- cleans out lower GI -- correlated with ¯ cancer of colon
b. soluble fiber (e.g. gums, pectins)
- digestible
- ¯ cholesterol concentration
- binds bile salts
- bile still needed to break-up fats
- more cholesterol converted to bile salts
- cholesterol pulled from bloodstream
2. dead bacteria (20%)
3. fat (10-20%)
- steatorrhea = fat in stools
4. inorganic material (10-20%)
5. undigested proteins (10%)
“Poo” does not “mix very well” at any other place in the system than in the colon. Even with a blockage, you need to explain even more than some other posters have said. Food and indigestibles move through the intestines because of peristalsis, a contraction of both circular and longitudinal muscles surrounding the intestines. It is a one-way process. The best analogy is to think of a farmer’s fingers pulling down along a cow’s teat to squeeze out milk. I can’t envision pushing the milk backward up into the body.
For “poo” to move backward up 15-30 feet of intestine, you have to assume both that peristalsis has failed entirely, so that there is no downward motion at all, contrary to normal activity, and that some form of upward squeezing pressure has been generated. More than that, you also have to find a way for the “poo” to travel the wrong way through one-way valves between the small intestine and the large intestine and between the stomach and the small intestine.
As our Good Doctor says, a direct connection between the colon and stomach may allow passage between them. But “poo” can’t travel backward from the colon to the mouth. I want a much better citation than second-hand tales of hospital horror stories.
Mash it between your fingers and then smell. If it’s a tonsolith, you’ll know it.
I swear by The Oxford English Dictionary. Then again, it’s a subscription service.
sorry Exapno Mapcase I was just speculating. but then again if you injected a dye into the milk of a cow near the end of the teet, then sealed up the opening. When farmer John when to mike betsy, and started parastolisis of the teet, and gave up, I bet if you were to kill the cow and check out the milk the dye would have spread up, though farmer John has been doing this for years and is an expert at getting milk down (and out) 
Also though I said that bacteria are added, I didn’t mean that, just food was moved into a place if these bacteria.
I did not know of a valve between the S.I. and the L.I.
I know of a case where this happened. My friend (not friend of friend) was serving in a mission school in Africa, in a small village. (Hmmm. Was 15 years ago, I can’t even remember which country it was. Geez, that makes it sound even more like an urban legend. But it’s not, honest.)
He started having stomach pains, and then his gut started swelling up over a few days and the pain become more intense. Following much vomitting, they began a 4 hour Jeep drive over bumpy roads to get to the nearest hospital. When he finally got to see a doctor, the doctor told him to lie face down on the gurney. He was reluctant to do so with his distended abdomen, but ended up doing so.
That was all it took. He says he now knows what someone goes through when told to “Eat $#%^ and die.” Of course, since he went into emergency surgery right afterwards, he was in no position to judge the precise makeup of his vomit as to whether it was actually feces or not. He thought that it was close enough to feces for his “tastes”.
Turned out to be a section of small intestine that had twisted and died, and he was lucky he had gotten to a hospital when he had.