If some people eat one food not on their plan, their failure to be perfect sets in motion a psychological chain of events that leads to frustration and the inability to get right back on the plan. The all-or-nothing mentality sets in and BAM, they're back to nothing. But it doesn't have to be this way. 100% nutritional discipline is never required for optimal progress. The difference, in results, between 90% adherence to your nutrition program and 100% adherence is negligible. So allow yourself the extra 10% wiggle room. This will allow you the freedom to eat a few extra things not on your menu without the guilt and subsequent crash.