Please, if a customer politely asks if there's a slight possibility that a certain item is in the back, do not cross your arms and smugly say "No" with a smirk on your face, and then turn to your co-worker and mockingly say "DUH" and laugh when you think I'm out of earshot. Maybe you were aware I was within earshot, maybe you were just being an ass, I don't know. Either way, you were being a first-class prick and you deserved the complaint I lodged with your manager. The fact that I was polite to your manager about your actions was far more than what you deserved.
Believe it or not, mistakes do happen. And even if you know for sure that you don't have it in, a simple "I'm sorry, we don't" will suffice.