I had a great dentist for years. however, he missed a smelly decaying tooth under a crown for 5 years. how?

Good question: Great dentists don't go bad. 50% of crowns fail within 10 years, decay around the crown is a common reason. The decay is caused bacteria that multiplies in extreme numbers creating dental plaque. This concentrates acid against the crown/ tooth joint. A break then occurs, allowing an infusion of bacteria between the crown and tooth. Only when large does it show on xray. The smell is bacteria.

Bad stuff hides: Sometimes it is very hard to tell if bad things are happening under crowns, because we can't see under them, and x-rays don't show under them either. If the only examination you've had in years is the quick one when you get your teeth cleaned, it's time for the dentist to treat you 'like a new patient' and do a thorough examination like she/he's never seen you before.