The hearse comes to a abrupt stop and the rear door flies open allowing the casket to slide out the back and onto the ground. There are two major errors. First, the door would not come unlatched. Second, due to inertia, everything, including the casket would slide forward, not backward.
At the funeral of Harland, David walks away from Maddie, revealing Cybil Shepherd is wearing New Balance sneakers, yet every time she walks on concrete, the sound effects are clearly of high heels. Later, David walks away from Maddie again and you can now see Cybil Shepherd is in fact wearing high heels.