What I expected to happen was...:
Either one Piston blocks the other or they extend after each other

What actually happened was...:
that the one piston got dublicated

Steps to Reproduce:
1. Place two sticky pistons facing upwards
2. Put one piston facing sidewards on top of one of them
3. Power the two lower pistons and the air block above the sideward-piston directly with repeaters at the exact same delay
4. Wire it to a button and press it

but somehow it also matters how the blocks below the redstone are placed. The setting in the pictures works.