>>Alejandro Sosa recently started Thread#1548796. It was started in a forum to which I subscribe.
>>
>>This morning I created a reply in that thread. After doing a manual global refresh, I could not find the reply I had just created. It seemed to have vanished, or perhaps was not saved properly.
>>
>>I was about to start a thread here, asking if you have had a glitch with the site. But, then I made a lucky guess, that maybe the thread had been moved, to a forum that I hadn't subscribed. That is exactly what happened - the thread was moved to the "Open Source" forum. After I subscribed to that forum, I can now see that thread.
>>
>>My suggestion:
>>
>>- If people have participated in a thread
>>- And that thread gets moved to a forum to which they're not subscribed (so their messages will seem to vanish)
>>
>>Then, could those people get some sort of notification:
>>
>>"Thread {Thread Number} {Thread Title} has been moved to {New Forum}. You are not currently subscribed to {New Forum}. If you would like to continue to participate in this thread, please subscribe to {New Forum}".
>>
>>Another possibility would be to automatically subscribe those users to the {New Forum}, in that particular circumstance. That could be combined with a notification similar to the above - some people might get annoyed if they get subscribed to new forums without notification.
>
>I was wondering why that technical thread was posted in Chatter. I actually didn't see it as I have many categories filtered, but sometimes I use Calendar option to check, if there is anything interesting going on in Chatter.

Isn't it you (maybe among others) who moves threads around without telling anyone? I think Al's suggestion is a good one.