Happened once this year, once last year. Stumbled on a loose rock on the farm track that leads up to our house; a moment's inattention, and thumped my kneecap - slowed me down for a few weeks (I 'Nordic Walked' the Parkrun the following Saturday) but I seem to have recovered now.
Last year's was more embarrassing - I was on a very quiet minor road when a delivery truck appeared and slowed down so that the driver could ask me directions. Distracted, I put my foot into one of those little drainage channels that are dug to drain water off the road, and next thing I knew I was face down on the tarmac, trousers shredded, blood pouring from knee, right in front of the driver! Was still able to give him directions to the farm he wanted, even though I was 25 miles from home.