Player D is in turn, the chips need to stay in the pot, I would let him forfeit the $600 and fold his hand. The consequence for being inattentive has been paid and no action out of turn is created by allowing him to fold without further penalty.
If this player had stated, "I call 600", I suspect that Thomas McGee might reasonably argue that this falls under the "gross misunderstanding of the bet" clause. As such, a case could be made for allowing the player to retrieve all chips and fold without penalty as long as no one acted behind him.
I hesitate to tell you how I would feel obligated to rule if the player had tossed in the chips and two players called the 6000 behind him. At that point I would make the short caller (D) put in the full 6000. The players behind D have a very reasonable expectation that any chips put into the pot represent a call. If we fail to rule it that way, everyone would quickly learn to trickle a few chips out, find out how many players call or even raise behind us, and then claim to have misunderstood the amount of the bet-hoping to minimize the loss.
