I know this is old but basically I think it can only be one point if the offense fumbles the ball while attempting a PAT (I think the ball can be kicked but there has to be an offensive possession by the kicking team in some way for it to happen) or a two point conversion and the ball goes out of bounds with the defense not having possession of the ball after the offensive possession or purposely knocking the loose ball out of bounds.
The league changed the rules a few seasons ago. A team can score 1 point if the resulting action would be ruled a safety, either on a point after try or a two point conversion. From NFL Operations, Rule Book
Article 2. Results of a Try
During a Try, the following shall apply:
(a) If a kick results in a field goal by the offense, one point is awarded. An artificial or manufactured tee shall not be permitted to assist in the execution of a Try-kick. (The conditions of 11-4-1 must be met.)
(b) If a Try results in a touchdown by either team, two points are awarded.
(c) If the Try results in what would ordinarily be a safety against either team, one point is awarded to the opponent.
(d) If any play results in a touchback, the Try is unsuccessful, and there shall be no replay.
(e) The Try ends when:
- either team scores
- the ball is dead by rule
- a fumble by either team is recovered by a teammate of the fumbling player