As it's easier to show with a video (would take longer to read a description), one is provided. Two related/similar bugs present.
Environment
Windows 7, unknown java
MC-13352
Piston retraction/extension bug with diagonal pressure plate
Resolved
Duplicate
0
153rd Duplicate of MC-108, please use the search function to see if your bug has already been submitted. Currently over 55% of tickets are being closed as duplicate.