The black plastic cover on my driver-side door sill has cracked and broken. I know it bears some weight when I slide into the seat, but I'm not especially heavy and I don't have that many miles on the car.
It is a common issue. My Roadster has a small crack towards the rear of each sill. I believe Tesla sells replacement sills in carbon fiber on their website. Al & Eds Autosound in LA also sell carbon fiber sills for slightly less. Also try eBay.
Someday I'll replace my sills with carbon fiber ones but right now I'm saving my pennies for the 3.0 upgrade.