Yeah, the last time I locked the rear wheel was downshifting into first while braking to avoid colliding with a guy using a leaf blower whom I couldn't go around on account of an oncoming car. It was 19 degrees F (-12 or so C?) and I had ridden less than half a mile, so the tires were like rocks. I was gentle with the clutch, but was using the rear brake a little and the tire still skidded. So now I try never to downshift into first with cold tires. Especially 19 degree cold tires.