I found a weird bug a while ago. Sometimes, when you power a double extender with pistons facing up with redstone going in to it, either the top or the bottom one will fire.

How to replicate:
1. make a 1 tick pulse shortener by placing redstone, a piston with a block on top next to it and a repeater behind that (as in the pic).
2. connect a redstone wire to the repeater and go one block out every other block and put redstone on top of that (like in the pic).
3. place 2 pistons on top of each other at the end of each redstone wire pointing out, so the wire goes into the pistons.
4. press the button.

Note: you won't always get a weird result, it's very un-predicatable.