I agree with all the posters that would give the player the option to fold, and requiring the chips already committed to the pot to stay in. I think the Rules are clear in this regard.
I am generally wary of enforcing the gross misunderstanding "exception" in any situation where the bet/raise is in plain view. It is one thing if a player had simply placed chips out in front of him that amounted to a call, and nothing else visually that would suggest that more was being bet, but it turns out that he announced "all-in". In some of these cases, I am comfortable enforcing the gross misunderstanding exception. On the other hand, if a player can plainly see an opponent's bet, the player considering a call is solely responsible for not interpreting the bet clearly. Poker is a visual game. It's up to him to make sure he assesses a player's bet correctly, whether that involves counting out a humongous stack of chips, or verifying the color of one or two chips.
Here's something that works for me, and I hope I don't offend anybody because I don't mean to. Whenever I'm faced with a decision at the table where I might be considering whether the "gross misunderstanding" exception should apply, I ask myself: "Would a deaf person have misunderstood the options that he faced based on what he has seen?" If this notional person looking at an opponent's bet would not have interpreted the bet as all-in, I may allow the exception if it seemed that the player did not hear the "all-in". If this notional person looking at an opponent's bet and what the opponent is holding behind still wouldn't be able to see those two 100K denomination chips hiding under the all-in opponent's hat or IPod, I might allow the exception. But if being deaf wouldn't affect the player's ability to accurately assess the amount of a bet moved into the pot, then I will not apply the exception. I know this all might seem a bit strange, but it works for me when I am pressed to make a quick decision.