Think of it this way: If they had inadvertently charged your debit card or credit card a few hundred dollars, wouldn't you expect them to credit you for funds you never used?
These kinds of things happen due to human error. Miskeying one digit is all it takes, but someone else is missing those $400.
On the other hand, if the IRS inadvertently credits you a billion dollar refund, grab it and leave the country.
