Here's a few glitches i've found so far:
When the lever in the picture is flicked and the signal strength changes the piston will retract and extract quickly.
This happens in a few cases it seems. Place the piston pointing south at x: -7, y: 56, z: 681 and set it up like in the picture..
1. They seem to be instant sometimes and somtimes have a small delay depending on placement.
2. Even though they are graphically lit, comparators don't output when indirectly powered.