I have found that taking a minute to rearrange tables in the QBE editor, so that join lines don't run behind other tables or cross each other, helps me spot errors more quickly and understand my relationships.  I like to make then short and straight, as much as possible.

In Lee's example, he may sell .38 cal firearms but not .38 cal magazines.  Likewise, he may sell .308 magazines but not the firearms.  A neatened relationship view might clarify that each instance of the calibration table needs to be unique.  

This also makes the "trouble patterns" easier to spot.