You shouldn't try to use the quote feature AFTER going back. Because of the nature of AJAX scripts and the way different browsers handle cached content with the back button, it's wonky and unpredictable. If you screw up a post and want to save your text, best thing to do is go back, copy the text, then click out to the main page or some other page of the thread (just any link that takes you off the cached page), then click back in and paste what you copied and resume where you left off.
It should be noted that this isn't a failure of the forum software, just a matter-of-fact consequence of modern web techniques and browser features.