Auto Advance to Next Field
Do auto tabbing a field break the Principle of Least Astonishment?
Yes, for the most part auto tabbing to the next fields (input) would break this law. Because auto tabbing to the next field is not common practice on the web and most desktop applications. This functionality would easily surprise and confuse most (beginner/novice) users working with the interface for the first time.
The only exceptions I can see for this rule would be if you are building a new application to replace an existing one with the same type of functionality already in place. Or if you are building an new application for a set of internal (power) users. Power users (users that work with any given interface for a long period of time) can quickly adopt to features like this.
Though if possible I would suggest even offering the ability to turn it on or off based on user preference it not remove it entirely. Of course that would be based on your application/interface requirements.