Apparently, he gets this a lot. From the official F.A.Q. on the excellent warwickdavis.com:

What was Val Kilmer like?

During the shooting of ‘Willow’, I found Val to be a very helpful chap. He has a great sense of humor and liked to shock. A good example of this occurred when we were about to fly off to New Zealand. I was seated in First Class, posing while wearing a pair of fake ‘Rayban’ sunglasses, when Val boarded the plane. He snatched the glasses off my face and snapped them in two. “Great”, I thought, “There goes my cool image!”. After gaining much amusement from my shocked reaction, Val produced two pairs of real ‘Raybans’ and gave them to me with the words “These are much cooler!”.