Is it ever OK to lie in front of your kids?

"OK, Tante asked me if we're going to Yaya's (Grandma's) on Saturday, but I lied and told her we had plans so that we can surprise her," I explained my elaborate scheme to my son. My sister was returning from a few weeks in Thailand and wanting to see my kids. I thought it would be cute if we showed up at the airport, when she'd be expecting my dad. (My dad is the airport pick-up master in our family.)

"You're just a liar," my son replied, astonished that I, his saint of a mother, would dare to cross the line. "Lying is bad Mama. You're not supposed to lie."

How do you explain to a child that sometimes lying is OK? I was completely stuck. After years of telling him that lying is wrong, how would I explain the nuances of a good lie vs. a bad one? And is the answer always black and white?