Pretty sure it wasn't Discord, who messed around here as he did with Fluttershy's dress, but he surely does like spotting ponies in compromising situations! XP

*"Dear princess Celestia,
Today I learned that a mare should always avoid pointing her butt towards dragons - even if he's just a baby one - unless she wants to get her rump scorched and singed.
A mare should also keep herself (as well as her rear) no closer to a dragon than their fire breathing range, especially when she's expecting a letter soon.
If the precautions listed above are decided to be ignored, she might wanna consider putting on fireproof panties beforehoof or buying a fireextinguisher for dealing with consequences.

Your faithful student,
Twilight Sparkle

P. S. - Would you kindly spare me a bottle of burn ointment for a week? I'd like to reduce the amount of days when I won't be able to sit"* xD

The alternative version without burnmarks on Twilight's butt and smoke can be found here: https://sta.sh/026yfxdohnt0