behavior before update: when a pumpkin/melon appeared under an observer. it powered a piston that swept the block away without powering the observer again.
behaviour now: piston makes the observer send another pulse as it retracts making it power again into an infinite loop.

(in the provided picture we can see the design of the mechanism as it used to be before. (in the front) and what has to be done now for it to not be a loop that would make the game lag (in the back)