When applying a credit memo to an invoice with different funds, the system tries to pull the intefund set defined in the "default" area at the top of the interfund page in Configuration. This becomes a challenge when the default set does not have the necessary entries needed that the interfund set on the credit memo does have. Before you can apply the credit memo, this makes it necessary to stop and change the default set temporarily if you do not wish to add the fund combination to the default set.