When Tyrion came to King's Landing, he'd just been appointed acting Hand of the King, and he was suddenly eager to prove that he was smarter than the previous Hand, Ned Stark. But last night's episode shows him facing the exact same fate as Ned, because he couldn't stop being too clever for his own good.
Tyrion's masterstroke was that he told three different people three different plans, to see which one got back to his sister Cersei: Littlefinger, Varys and Pycelle. Whoever told Cersei, Tyrion would know he couldn't trust. When Pycelle ratted Tyrion out to Cersei, Tyrion had Pycelle tossed in a black cell. And then Tyrion went ahead with his real plan, sending Cersei's only daughter Myrcella away to Dorne to be protected (or to be a hostage.)
There are a few key flaws in Tyrion's cunning ruse, however. First of all, he should have known already who he could trust: nobody. Second of all, the two people he came away deciding he could trust were Littlefinger (who framed him for murder, although he doesn't know that) and Varys (who just testified against him at his trial.) Third of all, he made an enemy of Pycelle, who is only too eager to make up crazy stories about Tyrion stealing every poison on Earth from Pycelle's stash.
This whole business of figuring out who you can trust is only slightly different than Ned Stark insisting on giving Cersei a fair warning before revealing her secret — in both cases, it betrays a misunderstanding of how the world works.
