In Discorded Ponies, a series of fanfics by alexwarlorn, Twilight states:

“I am NOT a Mary Sue!
Ahem.
1) If I was a Mary Sue I’d have been teaching my new friends what it meant to be friends when I first came to Ponyville instead of the other way around.
2) My spell to redirect the parasprite’s hunger wouldn’t have backfired.
3) My failsafe spell wouldn’t have failed.
4) Also I would have defeated an actually Ursa Major instead of pacifying an Ursa Minor.
5) I’d have won the Running of the Leaves race instead of coming in fifth place.
6) I wouldn’t have a nervous breakdown every time a plan of mine blows up in my face.
AND I’D HAVE ALREADY FIGURED OUT WHAT IS WRONG WITH MY FRIENDS!”

I rest my case.

Mary Sue, from the Latin butthurtis. Noun. A. To complain about changes to a character you don't agree with. B. When a character does something considered "OP", despite a lack of evidence. C. A common insult that has lost most of it's meaning through overuse and misuse.

Used in a sentence: OMG Twilight is OP'd cn u belife it & is totaliy a MARY SUE now!!!!!!!!!