Voldemort realised the danger in letting Harry read his mind -- Harry was getting way too much information about what Voldemort was doing, thinking, and feeling at that moment.
"So tell me, Harry," said Dumbledore. "Your scar...has it been hurting at all?"
Harry raised a hand unconsciously to his forehead and rubbed the lightning-shaped mark.
"No," he said, "and I've been wondering about that. I thought it would be burning all the time now Voldemort's getting so powerful again."
He glanced up at Dumbledore and saw that he was wearing a satisfied expression.
"I, on the other hand, thought otherwise," said Dumbledore. "Lord Voldemort has finally realised the dangerous access to his thoughts and feelings you have been enjoying. It appears he is now employing Occlumency against you."
This precedes the events of OotP, where because of this "mind-reading loophole" Harry is able to save Arthur Weasley and reveal Voldemort's very apparent interest in the Department of Mysteries.