"Shortly after becoming an alicorn, Twilight immediately set out to put her new power level to the test. After swapping the races of two of her friends with little effort, she began digging through a dusty old spellbook in the library. It was a pretty hot day outside, so finding a spell that 'cools ponies off' was all Twilight needed to read about it before giving it a shot. About 10 seconds later, this happened. Suddenly super-naked, the newly crowned Princess begins to realize how the prank spell actually 'cools ponies off"

TL;DR:she fucked up a spell