I just got a glimpse of this, and it may either be a blooper or a mistake on my part! When they first show the grave (4-5 minutes into the episode) there is no tree or bush behind it. Near the very end, they show it again, and now there is a tree or bush behind it.  Thanks Renewed Fan! 
 

 The date on Sam Elder's marker for his grave was 1871 which is far too early (Mark would have been born pre-Civil War. In the 2nd season episode 'Martinet' episode #83 the date on the grave was 1883 or 1884.  In the last scene of 'Boomerang', there is a blooper - Johnny Crawford almost falls down when the buckboard starts moving. 
Thanks Renewed Fan!