By placing an upwards-facing sticky piston with a sideways-facing observer on it that powers a block with redstone dust under it that will power the piston, one can create an infinite loop of piston extension and retraction, without the redstone dust or observer ever appearing to be powered. I believe this to be a bug because it does not conform with quasi-connectivity and the 1-tick pulse from the observer should cause the piston to not retract the observer after extending. The piston can only be stopped by breaking one of the aforementioned components or updating it while it is extended. The redstone dust does not even have to be in an orientation where it should be able to power the piston. One can start this loop by powering the piston or making the observer power the block.