When Rainbow Dash becomes Captain of the Wonderbolts, tradition mandates that Spitfire tell her the story of the Failure of Swiftwing, the first captain of the Wonderbolts.
Pull up a cloud; it's storytime.

After her first time donating blood, Fluttershy saw something that convinced her that Nurse Redheart is a vampony. Teaming up with Twilight Sparkle, the two try to prove and disprove, respectively, the vampony status of Nurse Redheart. Of course, all this must be done all while trying to avoid raising her suspicions.
Which, of course, cannot end well.