Yikes, that's frustrating. I've had SMC mysteriously forget message text I've spent a lot of time on, especially if I switch briefly to another tab in the browser or use another application. For protection, I've learned to hit Ctl-A/Ctl-C to quickly select and copy the text to the Clipboard before doing ANYTHING else or clicking on any buttons, even the Post or Preview buttons. If I use another tab to find a link or text to copy and paste in a message it would overwrite what I have in the Clipboard so I open Notepad, paste the message so far there, then get my link. That way if the message disappears I have a copy at hand.
BTW, the dumbest mistake I've made (dumb because I've done it twice) is compose a message then for some reason click the "+Reply to Thread" button instead of the Post. Sometimes "back" doesn't. The second dumbest thing I've done to make a message disappear is do a preview then get distracted and forget to actually post it. :-( Later I wonder, where is that message!
JKJ