Today I used a new analogy to describe 3-way matching of a Purchase Order, Goods Receipt and Invoice Receipt to my best friend. Let's say we have a Purchase Order for 5 apples and 5 oranges. 5 apples were delivered, and an invoice was sent for 5 oranges. In this case, there's a partial match between Purchase Order and Goods Receipt (5 apples); and a partial match between Purchase Order and Invoice Receipt (5 oranges). However, there's no match between Goods Receipt and Invoice Receipt so 3-way matching has failed.

The moral of the story is: you don't pay for oranges when you get apples.