In the book 'Dead Beat', he needed information from Bob's days with Kemmler, and asked for it, despite Bob's warnings..
I snorted out a laugh. "Bob. You never forget anything."
"No," Bob said. His voice shrank into something very small. "Unless I want to, Harry."
... And a bit later ...
"If it hurts," I said, "I'll stop, and you can forget it again when we're done talking."
"But—" Bob said.
"It's a command, Bob. Tell me."
It was a bizarre sight. The cloud of lights shivered for a second, as if in a trembling breath of wind, and then abruptly just shifted, flickering to one side as quickly as if I had been looking at it with one eye closed and suddenly switched to the other.
"Kemmler," Bob said. "Right." The lights came to rest on the other end of the table in the shape of a perfect sphere. "What do you want to know, wizard?"
There was a flash, and a pair of white eyes formed in the glittering cloud of red points of light. They weren't pleasant. "Shall I show you the start of the path?" Bob's voice said. "Death, Dresden, is a part of you. It is woven into the fabric of your being. You are a collection of pieces, each of them dying and in turn being reborn and remade."
"Too late!" crowed the spirit. It shot into the air, swirling around over me, gloating. "Whatever you have done to my thoughts, the master will not be pleased that you have meddled with his servant."