A few weeks ago the new Shout Factory DVD "Soaring Over Equestria" was announced for release on August 2, 2016. This DVD was planned to contain the as of yet unaired episode "Fault In Our Cutie Marks", but this episode has since then been removed from the listing and been replaced by "On Your Marks".
"The Fault In Our Cutie Marks" was originally intended as episode 9 of the sixth season, but was moved to air at a later date. The reason is was replaced on the DVD is probably to prevent an episode appearing on a home release before it airs on Discovery Family.
Find the updated DVD listing over here.