Here's the mistake Arya Stark made in the 'Game of Thrones' finale

Warning: Spoilers ahead for season six of "Game of Thrones."

Arya Stark delivered a surprise - and glorious - act of revenge during the season six finale of "Game of Thrones."

Using the face of a serving girl, presumably from the Hall of Faces, Arya arrived at the Twins and exacted her revenge against the Freys by murdering two of Walder Frey's sons, baking them into a pie, and serving it to him. She revealed herself as a Stark before slitting his throat, but even before she unmasked herself, she slipped up a bit.

As Arya delivered the pie to Walder Frey, she referred to him as "my lord," which is how highborn people address each other. If she was a true commoner, she would have said "m'lord." This is a lesson Tywin Lannister tried to teach her back in season two.

Arya was posing as a young boy named Arry when, as a captive of Ser Amory, she met Tywin at Harrenhal. He realized right away that she was a female and made her his cupbearer. While she was serving him, she slipped up and called him "my Lord."

"Lowborn girls say m'lord, not my lord," Tywin told her. "If you're going to pose as a commoner you should do it properly."

She didn't quite remember that this time, but it didn't affect her avenging her family. Though, it wouldn't be a bad idea to heed Tywin's lesson in the future, just in case.