I have to disagree. It *is* slipping to have two slices of pizza or a few cookies if you aren't supposed to be eating them. No, it's not the end of the world, and two cookies are better than a whole bag, but it can start you on a downhill slide. I think a better "treat" would be non-food related, like treating yourself to a movie, manicure, etc. Otherwise you could start "treating" yourself right back into weight gain. And I don't want to send the message to my kids that food is a reward (treat), or a bribe, threat or punishment (the "no dessert if you don't eat your dinner" or "I'll buy you an ice cream if you behave" type thing).
I "slipped" a few days for my DS's birthday and I didn't lose all week. The next week I had to really work hard to make up for it. For those of us who are lo carb - after eating carbs, it makes you crave them. A slice or two of pizza leads to more pizza which leads to more carbs, and after getting back on track it takes a couple days for the cravings to go away. It is not fun. Better to say "no", save the guilt and calories/carbs, and pat yourself on your (thinner) back.
I try to repeat that old mantra "nothing tastes as good as thin feels". LOL