Penalties and Direct Free Kicks are unassisted, though some have argued in the past that the person who draws the foul should be credited it's never really gained traction in the official statistics. Similarly you don't officially get an assist for an own goal although you could probably mount an argument that in many cases a good in ball creates a goal and as such perhaps it should be. If the ball hits the post and ricohets out with a significant change in direction there's no assist, same if it comes back off the keeper with a significant change in direction. Also if a player disposseses an opposition player and scores directly, no assist.