Book readers are a fickle bunch. In the book thread they talk about how shocking this chapter was, but in this thread they are amazed that TV watchers did not see it coming.
Not that they didn't se it coming. It's that they didn't recognize the song and its meaning. I'm impartial as I haven't posted on it and need to watch the last 3 episodes still.
Thats a lot to ask of a tv viewer imo. First you have to recognize the tune - its catchy, but its not getting a lot of radio play these days. Second, you have to recognize that its the Lannisters song. The second might have been easier to do - but you had to be paying attention to some seemingly benign lines going back to the Blackwater episode where Bron said he learned the Lannisters song by fighting with Lannisters to Cersei talking about the meaning of the words.
As others have pointed out, there were enough other visual clues that
something was going down - door being closed, Bolten wearing his chainmail, Cat looking around nervously. So, I don't think missing the song as a clue was a big deal.
I did think the show glossed over a big point in Westeros customs and unwritten rules - the bit about hospitality. A couple of episodes ago, Lady Cat warned Robb about making sure they are offered (and take) food and drink as soon as they get to the Freys - and at the beginning of this episode we see Robb eating and drinking and being offered the protection of hospitality by Lord Frey.