TL;DR: Sansa Stark thinks the Hound kissed her, but he actually didn’t (e.g. the “Unkiss”). The mismemory could signify Sansa’s attraction to the Hound.
Sansa Stark has been established as an unreliable narrator, and perhaps her biggest lapse in memory — at least that the readers are aware of — is the “UnKiss.”
So what is the UnKiss?
On the night of the Battle of the Blackwater for King’s Landing, the Hound visits Sansa in her room, bloodied and frantic. Sansa thinks the Hound is going to kiss her and she closes her eyes hoping for it to end, but nothing happens.
The blood masked the worst of his scars, but his eyes were white and wide and terrifying. The burnt corner of his mouth twitched and twitched again. Sansa could smell him; a stink of sweat and sour wine and stale vomit, and over it all the reek of blood, blood, blood.
“I could keep you safe,” he rasped. “They’re all afraid of me. No one would hurt you again, or I’d kill them.” He yanked her closer, and for a moment she thought he meant to kiss her. He was too strong to fight. She closed her eyes, wanting it to be over, but nothing happened.
-A Clash of Kings, Sansa VI
However, later in A Song of Ice and Fire, Sansa misremembers the events from the night:
Sansa wondered what Megga would think about kissing the Hound, as she had. He’d come to her the night of the battle stinking of wine and blood. He kissed me and threatened to kill me, and made me sing him a song.
-A Storm of Swords, Sansa II
It’s not surprising that she misremembers the night. There was a battle for the city raging outside, and Sansa did not know what would happen to her, no matter who won. Then she thought she was going to be kissed against her will — or worse. It was a harrowing event during harrowing circumstances.
But, according to George R. R. Martin, this misremembered event could play a role later in the series.
“You will see, in A Storm of Swords and later volumes, that Sansa remembers the Hound kissing her the night he came to her bedroom… but if you look at the scene, he never does. That will eventually mean something, but just now it’s a subtle touch, something most of the readers may not even pick up on,”
-GRRM, per Westeros.org.
What exactly it will mean remains to be seen, and there’s not too much to go on when it comes to speculating about it.
Why might the UnKiss matter?
However, one theory, originally posted on asoiaf.westeros.org, is that the mismemory is not a mismemory at all, but a fantasy of Sansa’s — perhaps one that she herself is not even fully aware of.
“For a mismemory to be considered as such, it has to come out of the mental realm into the realm of reality, otherwise it’s fantasising, pure and simple,” the original theory reads. “Thus, as far as Sansa’s imaginary kiss remains in her thoughts, it’s only her imagination at work.”
In other words, the significance of the UnKiss could be that it subtly represents Sansa’s attraction to the Hound. He was brute, and loyal to Joffery (often to Sansa’s detriment), but he was also kinder to her than almost anyone else in the Red Keep.
If the attraction exists, it could also be reciprocated. The Hound admits to Arya Stark in A Storm of Swords that he wishes he had done more to Sansa that night in the Red Keep — although he was vulgar and crude in this admission.
Nonetheless, with the Hound still potentially alive, this could set us up for a Sansa-the Hound relationship in the future. We’re not exactly rooting for this theory to be true — it’s not the type of relationship anyone would hope for, especially given everything Sansa has already been through — but that’s the speculation of the UnKiss.
The true significance of the UnKiss — if there really is any significance other than GRRM establishing the fact that his narrators can be untrustworthy — remains to be seen.