Found this weird bug on bukkit/spigot with a friend and apparently it's also there in single player.

When powering a piston through a block with a piston beneath it, the piston either spits out the block or not (see pics & how to reproduce)

How to reproduce with redstone dust:
-open creative world
-place 1 lever with 4 redstone dust surrounding it
-place a piston with a block on top at the end of every redstone dust
(like in the picture)
-place pistons next to the blocks you just placed (like in the picture)
-flick lever
notice, this is coordinate and direction dependent (i think)

How to reproduce with repeater/comparator:
-open creative world
-place 1 lever with 4 repeaters/comparators surrounding it
-steps 3,4,5 from redstone dust (but without the dust)
notice, this is only direction dependent (i think)

if nothing weird happened, try it on a different spot.

oh and i couldn't find a bug-report about this.