Sticky pistons behave unexpectedly when unpowered and powered again after 1 tick, if they have a slime block and another block (slime or other piston-moveable) in front of them; They fail to extend entirely.

In picture n°1, you see the expected behaviour, no bugs here.
n°2 shows the bug. To make this happen, one needs to turn the lever on once, and turn it off again.
As can be seen in n°3, the bug only happens when a second block is in front of the slime block, not when it's attached otherwise (i.e. at the side/top/bottom).
The bug also affects downwards- and upwards-facing pistons, see n°4.