A Game of Thrones
How we cite our quotes:
"Lord Petyr," Ned called after him. "I... am grateful for your help. Perhaps I was wrong to distrust you."
Littlefinger fingered his small pointed beard. "You are slow to learn, Lord Eddard. Distrusting me was the wisest thing you've done since you climbed down off your horse." (26 Eddard 5.81-82)
This is like the classic philosophical riddle: if a man tells you that he's a liar, can you trust him? How do you read this section? When Petyr says that he shouldn't be trusted, does that make you trust him more or less? In any case, we see that Eddard is starting to trust more, which sets him up for more (and worse) betrayal down the line.
"Tyrell had to know the mare was in heat," Littlefinger was saying. "I swear the boy planned the whole thing. Gregor has always favored huge, ill-tempered stallions with more spirit than sense." The notion seemed to amuse him.
It did not amuse Ser Barristan Selmy. "There is small honor in tricks," the old man said stiffly.
"Small honor and twenty thousand golds." Lord Renly smiled. (31 Eddard 7.89-91)
Littlefinger, Selmy, and Renly are chatting about Loras Tyrell's trick with the mare in heat that upset Gregor's horse. You can see a generational gap here, with Selmy taking the stance that you shouldn't cheat, while Littlefinger and Renly seem to believe that winning is more important, even if you have to betray your morals a bit to get there.
Tyrion shivered. Now there was a nasty suspicion. Perhaps the direwolf and the lion were not the only beasts in the woods, and if that was true, someone was using him as a catspaw. Tyrion Lannister hated being used. (39 Tyrion 5.49)
When we're talking about betrayal and manipulation, revenge is not fair play: the people who love doing the manipulating sure hate being manipulated. (We can easily imagine Petyr or Varys making the same comment as Tyrion here.) But this comment by Tyrion lets us know that some master betrayers might be off-screen. After all, we don't get to see Varys or Petyr through their own eyes. And who knows what else we're missing?