[19Yzerman19]

Not the king, the prince - prince Rhaegar. Remember (this has all been mentioned at least tangentially in the show), Robert's Rebellion, the means by which the Targaryens were overthrown, was basically started at a tourney at Harrenhal that was won by Rhaegar. It's customary for the winner to crown a "queen of love and beauty". Rhaegar was married to Elia Martell (the one Oberyn is currently complaining about being murdered at Tywin's order in the show), so you'd assume he'd give his wife his favour. Instead, he gave it to Lyanna Stark, who was supposed to marry Robert Baratheon. Lyanna disappeared a year later and it was assumed that Rhaegar had kidnapped her, triggering the rebellion.

So I guess it could be considered a spoiler in that Jon Snow's heritage could be important at some point. Especially given the mystery surrounding Lyanna's death. Here's a passage about this from the book that would have occurred during the first season when Ned gets attacked and has a fever dream.



Incidentally, that brings up a neat bit of trivia: when Benioff and Weiss pitched Martin about doing his series as a TV show, they talked about the books. Martin's litmus test for whether or not these were the right people to do the show was asking them who they thought Jon Snow's parents were.