How do I roll back an article?

You can always roll back to any previous save.

Tools >> Logs >> Article Logs >> click a Title

You are presented with a side by side view of firstly the log record you just clicked on and to the right is a copy of the current article - can be very helpful to "regrab" a snippet from the past or you can click "Restore" at the bottom of the historic copy.

If you save there is always a copy - I sometimes start an article as a pre publication article (temp, its id or filename will stay the same when its set live) and splat buckets of stuff in there - I title it SPLAT - "Article title" - then I open up a new tab and have the splat page open as I refine or rewrite the contents and get it into order.
I get an access denied message. 
The usual thing that gives access denied is an incorrect username or password. If the browser will not present the login box again you can in (MS IE) close all browsers and re-open, or in (Firefox Miscallaneous Clear private Data Clear Http Authentication) re-open the current page.
IE users should also check they have the following set. 
TOOLS >> Internet Options >> Privacy >> Advanced -- set "Always allow session cookies" to ON (ticked) 
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