So you're cruising right along, your Model S set on Autopilot, when suddenly the car says it's time for you to take over: Radar visibility has been reduced.
This is the story of redditor Redebo, who was driving across the Arizona desert when is car gave that very warning. Autopilot is not infallible and will turn over control to the driver should it encounter a situation it cannot handle — and the disabling of the forward-facing long-range radar is such a situation.
So what is it that took out the radar? An impact with a giant moth whose carcass decided to hang around in front of the radar window. The driver removed the insect with a gas station squeegee and continued on his way with full Autopilot functionality restored.
On a serious note, it's worth considering that Autopilot sensors do need to be kept generally clean to continue functioning properly. Between the front-mounted radar, the camera at the top of the windshield, and the dozen ultrasonic sensors around the body, there's a lot of sensing going on here. General everyday dust and grime won't have too much of an impact on Autopilot's ability to function, but should your Tesla end up covered in winter road salt, mud, or locusts, there's a chance that Autopilot might not feel up to the challenge of driving with one eye out.
Besides, keeping your Tesla nice and clean means that it'll be that much more beautiful out on the road.