A batter-runner is out when:
Here is Rule 8, Section 2, Article 5 from the Rules Book
The batter-runner shall be called out when: She runs outside the three-foot (0.91m) lane and, in the judgement of the umpire, interferes with the fielder taking the throw at first base (there must be a throw; however, the batter-runner may run outside the three foot (0.91m) lane to avoid a fielder attempting to field a batted ball. A runner is considered outside the running lane if either foot is completely outside the lane and in contact with the ground.
"if either foot is outside the lane and in contact with the ground" means even one step outside the lane and in contact with the ground. On a bunt play out in front of home plate that should be the key thing a plate umpire watches for on the play.
Here is a crazy situation for you to contemplate. The bases are loaded when the defensive coach asks for a defensive conference. The coach meets with the defensive players in or near the circle and the three base runners hustle over to the third base coach's box area to confer there. When the plate ump breaks up the defensive conference, the three base runners head back to their bases, or at least they thought they had. The next batter singles in the runners from second and third. The defensive coach then appeals that the baserunners had actually gone to the wrong bases after the defensive conference. The runner who should have been at second was at third and the other runner was at second when the ensuing batter singled. Figure this one out!