The only failures are with people that want to reinvent the paleo book and miss their bread and pasta and doughnuts. A little bit won't hurt.
Also the failures happen when the difficulties of social pressures of not eating the crap that arrives at potlucks get in the way of good etiquette. Or they cannot give up pasta night.
Eating out a lot and not knowing what kinds of oils are being used in the kitchen can sabatoge paleo. Melissa today is at a meet up at Chipotles in NYC protesting them using soybean oil when cooking their meats.
If someone decides to alter their eating habits strictly, they will not fail. But too many people complain of ailments such as arthritic hands or knees and when I propose they give up bread and pasting...they pasta...they say they could never do that. Or they complain of being overweight, but they will not give up their bread, cakes, and pies.
