This site allowed me to write a post, told me to sign in, and then threw away the posting I had written when the sign-in is complete. If you intend to induce rage in your customers, that is good desigh. Otherwise it is very much not. There is no reason you can not keep the text of themessage through the sign-in process.
Also, do any of your users understand how "topics" is different from "message tags"? I sure don't.