When a powered redstone block is placed diagonally from a piston, and the piston is unpowered, it remains expanded instead of retracting as it should. I have tested this bug in versions 1.0 through the snapshots for version 1.8. The issue, strangely enough, does not work in reverse. If the piston is unpowered to begin with, and you attempt to power it from diagonally, it will not expand. If anyone else is experiencing this issue and has anything to add to it, please do so!

I have uploaded a video to youtube that goes more in-depth with the issue. It can be found at: http://youtu.be/-HKJMXbFX5M