The dwarf ghost lady at High Heart prophesies for the Brotherhood Without Banners and Arya. Among passages about the deaths of kings, she says, "I dreamt of a maid at a feast with purple serpents in her hair, venom dripping from their fangs. And later I dreamt that maid again, slaying a savage giant in a castle built of snow"
It's likely these two sentences are about one person. We have seen
Sansa's hairnet of poison crystals accounts for the purple serpents
In a later scene, we see something hinting at the second sentence, but the symbolism is less clear.
Sansa builds a snowcastle at the Eyrie, but Robert knocks it over. Is he the figurative giant? If so, how was she predicted to 'slay' him? Was it by slapping him?