I have been having the same problems off and on for a week (?) or so - ESPECIALLY when I attempt to use Zemanta to insert a photo - everything goes wonky OR text disappears totally, leaving only the graphic (yes, I make sure I have an insertion point and that nothing is highlighted.)
I clear my cache & cookies, but that has never done anything for me but slow down the page load and eat more time when I need to retype cookie info for site memberships - I do have the sense to refresh, btw. I wonder if that advice is simply a sick techie joke like "type 'configsys' and write down everything you see while we're working on it" (oh yes they did! - not WordPress, btw, Accounting Big 5 techs to keep the secretaries busy - as if they didn't already have enough to keep them busy!)
TEMP FiX - if in edit, immediately use the "undo" arrow and try again. Several times, if necessary. It is an intermittent bug in Safari 5.1.2 (MAC) at least. Doesn't ALWAYS do it.
If your post blows on opening, scroll to bottom and select an earlier version to "compare" - then restore the one closest to what you thought you had. I also copy html to word, in case all else fails so I don't lose it ALL (and yes, if It's not html I use the little "word" box to insert).
ALSO - my graphics change size EVEN when I do nothing at all to the post "backstage." TEMP FIX - edit the post, reset the size manually (advanced) & resave - then go look, it should be close enough.
Sure hope you find and fix soon - it eats time by the tail AND I format specifically to compensate for Attentional deficits, so it is QUITE the problem when formatting changes. I have no choice but to waste time redoing.
Please let us know when we can stop wasting time with the workarounds. Thanks.