The Citroen AX has come up in our recent discussions. It has a sagging line at the base of the side glass.
In this case, the curve makes sense whereas it didn’t on the Mercedes E-class coupe or the Colt 1400 that started it all. I think the feature might be a very standard touch that, for a time, became one of those things a designer always does: link the side glass to the front windscreen.
Here it is with the sag removed somewhat. The line is horizontal from the A-pillar to the middle of the rear door.
Driven To Write 
The car that first irritated me with its sag was the 1976 Fiesta.
This sag, while it made complete sense, was one of the AX’s more irritating design details to my eye. I’d have preferred a straighter line and probably an offset between front and side glass. Or an overall more convincing shape like the monovolume style of an earlier prototype: