A Game of Thrones
How we cite our quotes:
"Lord Baelish, what you suggest is treason."
"Only if we lose." (48 Eddard 13.120-1)
This is a classic joke about treason (ah, how we love those classic treason jokes): it's only a crime if you lose. Again, we can see that Petyr isn't really trustworthy (and yet Eddard still trusts him, ugh!). But we should also note that Eddard Stark was involved in a treason that was successful: he helped overthrow the Targaryens. This is why some people still think of Robert as the Usurper.
As his men died around him, Littlefinger slid Ned's dagger from its sheath and shoved it up under his chin. His smile was apologetic. "I did warn you not to trust me, you know." (50 Eddard 14.58)
Well, this little comment doesn't really help Ned now, does it? There's something perfectly poetic about Petyr using Ned's own dagger against him. It's similar to the way that Petyr used Ned's own principles and trust against him.
"My words lied. My eyes and my arm shouted out the truth, but you were not seeing." (51 Arya 4.11)
Very soon after Petyr's betrayal, we get a tiny lesson in betrayal from Syrio Forel. According to Syrio, words can lie, but actions can't. (In this case, he was teaching Arya that her enemies won't announce that they're trying to attack her.)