Gaskets fail even on very good casing systems. This is a Rolex and it uses their "Oyster" brand casing to drive home how secure the watch movement is from the vagaries of day to day live traveling on some one's wrist.  
This one had a tiny bit of rust on the barrel arbour.  It's a mystery to me how it got there and nowhere else. Taking the gaskets out of their seats, cleaning the seats, replacing and or cleaning the gaskets and preparing them with silicon gasket grease is a good thing to have done as part of any servicing.
Good gasketing helps prevent rust damage.
rusted barrel arbour
Getting rust to stay away and stop creating more damage is difficult at best. More often than not its presence indicates the need for replacement parts.
close up rusted barrel arbour
One of the keys to getting in and out of a repair in a timely fashion when servicing machinery, is in staying organised.  
Clean screws lined up neatly