This is my opinion on what could have happened in the episode '28 pranks later '... (this is my very first story so there might be some mistakes)

Rainbow Dash has turned the entire town into cookie-eating zombies who want to teach her a lesson. Rainbow can't help but feel guilty, she basically killed everypony. But the problem is, she pays for a tragedy that didn't even happen...

Note: rated teen for Death only. And if you don't like character deaths, please don't read it.