What I expected to happen was, when a piston on top of another piston facing up would be powered with a redstone torch, only the bottom piston should extend and the top one shouldn't change its state.
What actually happened was that the bottom piston went to a state of extended and retracted at the same time, and the top piston became a "piston head".

Steps to Reproduce
1. Place two pistons facing up, one on top of the bottom one.
2. Place a block in the diagonal of the top piston (one back and one left/ right)
3. Put a redstone torch on the block, on the side of the top piston.