Whenever there are more than 1 observer within like maybe a 4 block area (not super sure, but maybe a general estimate), and they are connected to pistons, sometimes only 1 might activate. Sometimes observer 1 might activate when a flying machine moves, but not observer 2. Or vice versa. It seems completely random. However, it could just be messed up delay times that force one to go a bit before/after the other, but I do not believe so.