Did You Just Insult Me? (Crossing the Line)

In the retail world, a playful / joking manner can be endearing and make customers feel comfortable in your shop or when interacting with your business by phone or email.  But there is a line easily crossed (and of course it’s different for each customer).  Today the cheerful shopkeeper down the road pushed things a bit far IMHO.  Of course he’s looney so I’ll forgive him for it.

The question on my mind is:  How do you instill a culture of joviality in a company while also ensuring you stay safely away from that line?