Gone out kiting before with my prescription glasses on and suddenly realised it was a tad bright. Had to head back and fetch my prescription sunnies making sure not to fall so I didn't lose my normal glasses.
Not quite kiting related but I remember a mate of mine complaining to me that his new open back scuba fins were the most uncomfortable bastards out. When I took a look at them I realised he was using with with the plastic piece in them that holds it in shape. He'd only ever used closed back so he didn't know any different.
And then there was Matt's fall which is still one of the funniest wipeouts to date. (sorry, I had to post it again, Matt)