My boyfriend has been using this idea, for a long time. He makes bean soup, with just ham and beans.. maybe a onion, then it sits in the crockpot on warm until digested, by us. LOL. well today. I grabbed a bowl.. and noticed a faint sweet taste to the ham.. I have been digging for suggestions since, to know if I should throw it away or, continue eating it.. It still taste amazing. Just nervous is all. I am going to stick a thermometer in the soup to see what temperature, my crock pot is on, while on warm. Then, I will know if its cooking at at least above 140, safe area. We do this to chili too, and deer meat.. It actually, taste better the second day. Sign.. a bit unsure.. lol Rose Dubois
I fully endorse the "when in doubt, throw it out" doctrine, although I personally wouldn't consider a sweet taste to be doubt. As rfusca wisely points out, you can't taste or smell several kinds of contamination, and the ones that you can taste or smell, are usually sour, bitter, or generally pungent.
I suggest you have a look at the following question:
If your crock pot has really been on the whole time, then it might be perfectly safe. You need to ascertain what the "warm" temperature is. Assuming you still have it running, just stick a thermometer in and see for yourself.
Anecdotally, I've heard reports of crock pot "warm" temperatures ranging from 150° F all the way to 220° F. Very unlikely that newer crock pots are anywhere near that high end, but anything above 140° F is cooking temperature and will kill bacteria rather than allowing them to breed. So if your thermometer test reads higher than that, it's probably still safe even after all this time. I can't speak for the quality of something that's been cooking for 3 straight days, but there's no accounting for taste...
On the other hand, if your thermometer reads anything lower than 140° F, then you should throw out what you have and stop doing this from now on. A truly "warm" but not "hot" temperature like 100° F is practically a giant incubator, even worse than just leaving it at room temperature.
P.S. You should read the manual for your particular crock pot, because even though it might be safe for the food, I'm not sure if it's safe for your pot. Many of them come with recommendations not to use the warm setting for more than 4 hours, and don't really explain why. Possibly, they think the temperature might not be enough to guarantee food safety, although another (to me more likely) possibility is that it can stain, corrode, or weaken the pot.
How do you know, by taste, if anything is bad? You don't, you can't. The bacteria that grow and make you sick may be odorless and tasteless. "When it doubt, throw it out."