This just started happening to me recently. In July, on the way up Mt. Evans, was the first time, then on the way down a few more times. It happened again this past weekend (Mt. Evans again), and afterwards on 285 (which was pretty cool 'cuz it was at a much higher rate of speed ).

I've heard AirmenGoob mention that he can get his knee down in the first 6-7 turns in Golden Gate Canyon. Just curious if anybody else has this "problem".