Primal is a life style, not a diet. There is no reason to cheat on your life. If you want to eat a Snickers bar, for godshakes eat it and move on. It is about you, you don't need our permission.
If you want an occasional slice of pizza or a cup of ice cream, no big deal- just eat it. It's just feeding yourself. If you constantly want these things, I'd examine what you are eating that is primal really carefully. Are you bored with the primal foods? Not eating enough? Eating things you don't enjoy?
I've found that there are a lot of old foods I just lost a taste for. Bread, baked goods. I used to crave them, ate some and they kind of sucked. Good for me- now that I know they have lost their appeal, I don't crave them.