[Book Spoilers] Joffrey and valyrian steel

[King Tommen]

Not to mention that in order to make Joffrey the culprit, Martin has to shoehorn in motivation for him that doesn't really make sense. Joffey's a sadistic asshole who doesn't care about the welfare of other people so the only logistical reason he would have to go through with this would be to gain the approval of Robert, who he can't tell because the whole thing has to remain a secret. It completely falls apart if you think about it for more than 5 seconds, which Martin clearly doesn't want us to do based on how little importance he put behind the revelation.

The most obvious reason to send an assassin to kill Bran would be to shut him up so that he couldn't tell anyone what he saw (meaning it's either Jaime or Cersei). Why Martin decided to change this after the fact to something so silly is trying to add a twist for no good reason is beyond me.