Moat, we actually have much more than 10 3pi robots in this project, and most of them are still running fine (it's the fact that problems like this keep showing up that scares us). Since they are all running the same code (which is running a basic line following algorithm by default) as you stated, I don't think it could be related. And in the case of a blocked wheel for example, the wheel won't spin even when the robot is off.
Grant, the platform is flat and clean, so there should not be much interefering with the wheels. However, the custom expansion we designed holds a 6600 mAh battery that has a certain weight, but if this issue was due to the weight of that expansion, how would that problem appear only on a few of them?