[ozrien]

My experience with S4s is if the shaft isn't perfectly concentric the encoder might miss ticks. In other words if the encoder shaft is torqued at all, the encoder might miss a few ticks.

I found one bad encoder this year by just spinning the shaft with a drill and monitoring the output on an oscilloscope. With a fixed velocity you'd expect to see a pretty uniform waveform. On this bad one I saw gaps.