This was never a problem for us throughout the season. Never noticed any slipping.
Here's my attempt at a mechanical explanation of how we did it: The wheel is attached a triangle holding box thing which is attached to a stick which is attached to a gear which is attached to another gear or something which is attached to a stick which is attached to an absolute encoder.
Sorry, I'm a programmer.
The most crippling swerve issue for us was gyro alignment, as we used a field centric system. Sometimes the gyro would just go nuts, creating a phenomenon we like to call the "drunken apple"'
After putting the gyros in a protective shock-absorbing case, we didn't have any problems with complete gyro craziness, although gyro drift was still present and annoying. We have to zero the gyro many times during a match.
This is probably the biggest problem we've had with swerve, and I'm not quite sure how to fix it.