Have you ever tried to downgrade a modern browser? Every browser has an auto-update mechanism built in, usually running in a separate process (Google Updater is the Chrome one, iirc) that apparently was installed alongside its respective browser and given permissions to manipulate all of the files belonging to that browser at will. It is exceedingly difficult to remain on an earlier version because these processes are so hard to stop, and most browsers don't even give you the option to ignore automatic updates any more. Even if you do manage to, expect to be harassed every few minutes by popups helpfully offering to update your browser.

There's already a small section on this in the descriptive text. However, I think, in the broader picture this is more about aggressive optimization and nondeterministic behavior from the high level programming language point of view. Which may happen to any product. (E.g., at some point, Safari mobile had a problem, where some code it should have fallen back to in time critical moments was apparently already purged from memory.) The point is really, if you put all eggs into a single basket/browser, what may have been a minor trifle else is elevated to a completely different level.