Right after a new implementation, everybody is concentrating on getting the invoicing right, and paying suppliers etc. and they tend to assume that the GR/IR is taking care of itself. Generally, if everything matches, it does, the problem is that not everybody understands that the quantity must match exactly to clear the GR/IR account. A typical problem where the invoices are posted manually, could be users struggling to post fractions correctly, for example where using the standard goods receipt process for a service. Another problem, where invoices and credit notes are posted via an interface, is not being able to distinguish between a price and quantity difference and using e.g. a credit memo to post all corrections.