The current knee problem has no good story. Slipped on some leaves (or rocks under leaves) on some tight singletrack, slid partway down a hill on my side, and twisted it sideways.
My knee felt (relatively) good this morning, but it's swelling a bit so I might pass on the alleycat. I hit the ground pretty hard again today, but this was just trying something way beyond my skills.
I haven't been on my SS since I picked up this geared 29er... and I'm not so sure I'll go back anytime soon. At least, not until this knee is better.