AES-CBC decrypts to wrong output? Someone I know showed this to me, and I can't put a finger to why it's happening. Am I missing something?
They encrypt the text, and the output is really guessable. $u\to t$, $i\to h$ (it's one letter behind), the $?$ are just spaces.
They trick you with the IV. The IV must be the same for encryption and decryption so that the encryption has the functionality that is;
$$ m = D_k(IV,E_k(IV,m))$$
To surprise you, they modify the IV according to their desire but you failed to see that. This is the more general case of the Bit-Flipping attack on the CBC mode.
This attack works because the message has only one block and the change on the IV only modifies the first block. Nothing special there.