The most common problem is a valve at the back of the toilets that can be knocked loose and cause all of the toilets (which the Navy calls heads) in one of 10 zones to lose suction.

"That's just the nature of VCHT. It's a closed system and thousands of components ship-wide that fail daily. With one commode control valve failure, depending on the location brings down the entire zone"

The engineers looked at an old string of Christmas lights and thought "Yes. Yes this design will be perfect for the bathrooms of a multi billion dollar military craft that all of the crew members rely on wile out at sea. There's no possible way it could go wrong."