This is also an issue of 'What the hell did Cersei expect'. Why did she go into their prison on her own without guards? Was she so convinced of her own Tiwyn-ness that she couldn't do the basic math of "Organization that imprisons sinful people + Woman known to be incestuous = Imprisoned woman"?
It's simply in her character. Cersei thinks she's so damn clever, but she quite simply isn't. Hell, at least 2 people (Tyrion and Tywin) have told her that to her face and she still doesn't get it.
All she saw with the High Sparrow was an opportunity to get back at the Tyrells and had no thought to anything beyond that one short-term goal. Even if she did spare a single thought for anything beyond getting Loras and Margaery thrown in prison, she probably thought that she could control the High Sparrow. But she has realized that (just like Grandma Tyrell) the High Sparrow isn't tempted by wealth or power or anything like that. Cersei literally gave power to the one person she can't control.
One thing is certain, somewhere Littlefinger is laughing his ass off seeing how well Cersei is screwing up the capitol without any prompting from him (ruined brothels aside).
Pretty much what Steven said.
Cersei puts her faith in her position (Queen/Queen Regent) and in the famous/wealthy house she comes from. She thinks these two things alone will carry her over any challenges she faces but she's wrong.
It was explained last season that House Lannister's gold mines have run dry and that is why Tywin is investing House Tyrell in the crown. Cersei knew this and still chose to imprison publicly castigate and humiliate Margery and Loras. Tywin also told Cersei that the Crown owes the Iron Bank a substantial amount of money. Tywin explains that the Iron Bank can't be bargained with or cheated. However Cersei has every intention of doing the latter. Because she doesn't care. She thinks her title and famous name (Lannister) will spare her from any rebuke she might face.
Compare the 4 Lannisters.
Tywin: Leader of House Lannister. Served as Hand of the King to Aerys Targaryan (The Mad King) and Joffery Baratheon. He fought against Aerys, wiped out House Reyne and conspired with Walder Frey and Roose Bolton to destroy most of House Stark and their bannermen. Tywin commands respect across Westeros and in nations across the Narrow Sea (Volantis and Bravos).
Cersei: Queen to Robert Baratheon, Regent to Joffery, Queen mother to Tommen. She has power but never really ruled. As I explained above, she puts more stock in her title/position and the money attached to her famous name.
Jaime: Kings Guard and Knight. Put his faith in his swordsmanship and famous name. Jaime was more of a fighter, and has been considerably reduced since losing his sword hand. Which he lost trying to play his family name card with Locke, a Bolton bannerman.
Tyrion: Definitely the smartest and cleverest of his siblings. Due to his short size, Tyrion always relied on his wits and cunning to see him through things. It got him out of execution at the Vale, helped him when he ruled as Hand of the King, when he defended Kings Landing from Stannis, when he served as Master of Coin, etc etc. Tyrion plays his family name card also, but he's less reliant on it as say Cersei and Jaime were.
Found this on tumblr
House Lannister for you.
[yt]https://www.youtube.com/watch?v=WDJ9O4rPj6I[/yt]
] got it from a doll... and the Stone Men are covered in infected tissue. Presumably it has to be more advanced before his level of contact w/Dany would be a problem.
)
) who betrayed and lied to her so she went ballistic and banished him due to her hot temper. 
