On our 2015 robot, our driver would always complain the the left side of the robot would be slightly slower than the right. Throughout the entire season, we tried all different methods to track down the issue, and eventually came to live with it. At some point, a modification needed to me made that required taking of the entire left drive plate (a process far more difficult than it probably should have been) where we found that a nut had somehow gotten wedged in one of our wheels causing this
https://drive.google.com/file/d/0B3d...5hRWE1Njg/view
If we had kept that up long enough we'd have been close to carving through the entire thing.