This is an interesting one:
Law 27 - Appeals
7. Batsman leaving his wicket under a misapprehension
An umpire shall intervene if satisfied that a batsman, not having been given out, has left his wicket under a misapprehension that he is out. The umpire intervening shall call and signal Dead ball to prevent any further action by the fielding side and shall recall the batsman.
Now clearly this doesn't apply in the case of Bairstow, since he had no cause to believe he was out prior to leaving his ground - but I just wonder where the rationale for this law comes from? The only scenario I can imagine is a batter edging behind, initially taken by the wicket keeper, batman then leaves his ground believing he is out, only then for the wicket keeper to fail to complete the catch, then collecting the spilled ball and stumping the batsman who was already walking out of his ground..
Bairstow did leave his ground under a misapprehension, however his misapprehension was that the ball was dead. Why is a misapprehension of being out protected by law but not a misapprehension of the ball being dead? It could be argued that there is scope for amendment to the law here. I personally think that a batsman should only be given out leaving his ground when either in the action of attempting to play the ball, or attempting a run.