When powering a sticky piston (with a movable block in front) with a short pulse (e.g. monostable circuit, observer, etc.), it pushes the block forwards as expected but leaves behind an extra arm. This does not occur when a sticky piston retracts a block or when a regular piston pushes a block. According to
MC-5726 and prior versions, the sticky piston should only push the block forwards and then immediately retract.
https://gfycat.com/CheerfulAnxiousDipper (file was too large to attach)