I was trying to create a contraption to push dropped items towards me, but when I activated it, the piston would extend but it would not retract. It was placed on a regular cobblestone block with a repeater running the current into the cobblestone block.

Steps to recreate:
1. Place cobblestone block
2. Place sticky piston facing towards you on top of cobblestone block
3. Place redstone repeater in front facing towards the cobblestone block
4. Run wire from redstone repeater 2 blocks away
5. Place wooden pressure plate
6. Step on wooden pressure plate
7. Step off wooden pressure plate