Can a rotary incremental encoder, count more steps due to interferences
It is normal for a rotary encoder to go through the following sequence:

- A low     and B low
* A low     and B jittery
- A low     and B high
* A jittery and B high
- A high    and B high
* A high    and B jittery
- A high    and B low
* A jittery and B low


From a decoding perspective, one may either use the states marked with dashes or with asterisks.  If one uses the states marked with dashes, then when the encoder is near a switching threshold, it may appear to jitter back and forth between two positions.  If the decoder is designed correctly the overall rotation should be correct, since repeated jitters will cancel out, but a +/- 1 unit jitter may be annoying.  If one uses the states marked with asterisks, apparent jitter will be eliminated (once an input changes, all future changes on that input will be ignored until the other input changes) but backlash will be increased by a tick.

If the decoding logic isn't implemented properly, it's possible for the jitter to cause unwanted counts in such a fashion that they don't cancel, but the solution in that case is to fix the decoding logic.