While testing an automated farm design, I came across an issue where a piston would remain extended if it was originally powered by a Redstone block, and the Redstone block was retracted using another piston. Note that this only happens if the Redstone block touches the top (highest surface by y coordinate) of the piston.

Once the Redstone block is retracted, the bottom piston deactivates for less than a second, and quickly activates again. If this happens the only way to deactivate the piston is to destroy it.