You also didn't have the browser on split-screen (the Windows 7 one-window-on-the-left, one-window-on-the-right feature), right? That will do it too (well, it's also resizing the browser, lol, but I figured I'd point it out too).
I did catch this once, and I noticed what appeared to be a "flash of unstyled content", so Shadab is likely correct. I don't know how to workaround it though. The CSS is in the <head>, so it's not being loaded lated. TinyMCE is being init'd (on document ready) and its size is based on the size of the textarea it is based on. If it fires the document ready events without the full CSS being applied, then the width of the textarea will be wrong and the init will be wrong.
I don't see a feasible way to workaround this - I think we're doing as much as we can to not trigger this in the first place. Resizing TinyMCE is an issue, so it's not going to be easy to reset the width.
It looks like this may have been a long standing issue/approach in Opera. This blog does a pretty good job of discussing it: http://molily.de/weblog/domcontentloaded It also implies that HTML5 may implicitly say that it's wrong, though with the spec having changed, that might be correct.
As this goes away on the next page view, I'm going to have to tag this as a browser bug.