In Season 7, Episode 5 of Game of Thrones, Arya spies on Littlefinger and sees him being given a scroll by Maester Wolkan. She waits for him to hide it and then searches it out. If you're wondering what the hell that was all about, this post is for you.
Here's what the scroll says, courtesy of this eagle-eyed Redditor:
"Robb, I write to you with a heavy heart. Our good king Robert is dead, killed from wounds he took in a boar hunt. Father has been charged with treason. He conspired with Robert’s brothers against my beloved Joffrey and tried to steal his throne. The Lannisters are treating me very well and provide me with every comfort. I beg you: Come to King’s Landing, swear fealty to King Joffrey and prevent any strife between the great houses of Lannister and Stark."
Yep, it's the letter Cersei forced Sansa to write to Robb back in Season 1, urging him to bend the knee to Joffrey after Robert Baratheon's death and Ned Stark's arrest.
Sansa's letter is the first news Robb hears of Ned's arrest, and he's shocked. Maester Luwin immediately recognises Cersei's hand in the letter.
So why did Littlefinger deliberately plant it for Arya to find (as we know he did from the way he was creepily lurking at the end of the scene)?
You might remember in Episode 3 of this season, Littlefinger was VERY interested when Maester Wolkan mentioned the fact that Maester Luwin kept a record of every raven ever sent to Winterfell.
He wanted Arya to see it, and to make her believe that Sansa wanted to hide it from her, to fuel distrust between the sisters.
Littlefinger wants power, and he wants Sansa. He's seen how dangerous Arya is, and knows she probably doesn't trust him. Her returning to Winterfell (not to mention Bran and his "chaos is a ladder" comment) puts his plans of gaining power over Sansa (and through her) at risk. From Littlefinger's point of view, the more isolated Sansa is, the more vulnerable she is, and the more she'll rely on and listen to him.