On the Mac I'm running Safari 6.0.2 and Mac OS X 10.7.5 (Lion). The iPad is an iPad 3.
Sometimes I have an issue in my Inbox where though there'll be a PM but nothing in the field below it, and if I click the Reply to Private Message button I get a page where there's no PM either. I can go back and the PM will be there but still nothing in the field below. It only means that's unusual, not that I can't reply. It seems only to happen if the PM is lengthy or curiously if it contains a vertically written list.
The other editing issue is I think the same one others have mentioned in the Social Groups when I click Go Advanced I either get a message saying I haven't written enough characters even if I have or what I have written is gone. If I click the Back button everything is there. It only means I can't preview what I've written.