When E&OE appears on an invoice, it stands for errors and omissions excepted. It is essentially a type of disclaimer for clerical errors that may occur on invoices, but an error does not have to be present for someone to use this disclaimer.
There are times where a vendor or a business owner unintentionally makes a mistake on an invoice. This disclaimer serves to protect them should a mistake or oversight occur. This helps to protect the person who wrote the invoice in the event that they unintentionally make a mistake when adding up the final invoice amount or any other aspect of the invoice.