Yes, it happens now and then. When I notice it starting to happen, I copy my text if I haven't already, so that I can paste it in anew instead of having to write it all again. I'm not a programmer so I don't have any clue as to what the exact problem is, but I have frequented several forums over the years and I don't find it surprising that there are a few bugs or server problems or whatever. Shit happens.
(I do know, however, that there is a specific bug that makes a thread disappear from recent posts if a message is deleted from it, because it messes up some info about when the thread was last active. That bug is "solved" by bumping the thread again with a new message. So if one needs to delete a post, it's a good idea to remember the name of the thread and search for it in the search engine to make a new post. Then again, it is easier to just change the post than to delete it.)
