Sticky pistons, even on a long pulse, will occasionally not retract with the block they are meant to retract.

I have, in my survival world, a 10 level enderman farm. Each level consists of 10 sticky pistons with a stone brick on the moving face. I check it once a week and find that out of the 200 pistons which are on there, approximately 10 will have randomly glitched out, leaving the blocks behind.

To reproduce: Set up a lot of pistons which will be activated a lot. Unload and load the area (and world) at various times. After some time there will be some pistons which have glitched out.