A Sales Order has a Billing Schedule of 12 months (e.g., January 2013 - December 2013).
Invoice Number 1 was created (January 1 2013) but has to be credited because of some incorrect information (for example, incorrect quantity).
User makes a copy of Invoice Number 1 (using the Make Copy option) entered the correct information then saved it; and then credited (Credit Memo) the original Invoice.
User then goes back to the Sale Order > click on Next Bill.
The Invoice that is being created is for March 1, 2013.
The Invoice that should be created is for February 1, 2013 since the original Invoice is already fully credited.
As long as an Invoice is being referenced to the Sales Order, it would still be counted as part of the Billing Schedule even if the Invoice has already been fully credited. Using the Make Copy Feature of an Invoice still links the Sales Order to the Invoice that will be generated.
The system will check the Sales Order Record > Related Records tab and it will count how many Invoices are showing there and that will serve as its basis when the user tries to create the next Invoice against the Sales Order.
To avoid this scenario, instead of using the Make Copy feature of the original Invoice, user can just delete the original Invoice and then enter the correct one. No need to create a Credit Memo.
Another option is for the user to credit the original Invoice and then enter a stand-alone Invoice with the correct information (for January 2013) so as not to break the Billing Schedule in the Sales Order.