“Oh . . . well, if the caster finds herself with too much mass—if another pony interferes and is also teleported, or if her cargo is too heavy—the spell allows for it instead of fizzling. But, since the energy has to come from somewhere, it is taken from the local inertial equalizing portion of the spell. Thus, if a pony takes along too much mass, she hits her destination a lot harder than she meant to—unless she had the energy to compensate for that. I had that happen with Spike one time.”