I once "used" a knife to defend myself against myself. Just before I was about to buy another Fällkniven F1, I looked again at my two F1's, cut some cardboards with 'em and said to myself: I don't need another F1, especially not for the current price. I saved myself about 300 Euros as I was about to order the new one with white micarta handles.
Other than that, nope. I rarely carry knives except for when I am on survival trips. I've never had any confrontations of any sort for I had to use my knife.