Rainbow Dash's face suddenly gets sinister & she jets up into the air & flies around with Applejack still onto her tail, clinging for life. "Had enough?" She says stopping on a dime. "Mhmm" Aj says scared. Success, Rainbow Dash got her revenge for all the times Applejack grabbed her tail since episode 1.
They have a whole army of animators, some of which handle no more than 30 seconds of video per episode. There are lots of little details that occasionally swap back and forth depending on who happens to be animating it at the time. In this scene (near the beginning of Episode 3: Ticket Master) that blue stripe does not appear on Rainbow's tail. I also noticed the blue was not there, but I was attempting to keep it accurate with the actual scene, so I did not include the blue stripe.