In the same way drafts of posts are thread specific, drafts of threads are forum specific.
If you attempt to create a thread in the Bug Reports forum but don't post it, the draft will only show up if you attempt to create a thread again in that particular forum. I believe drafts are pruned after 24 hours.
The easy way around this is to just hit the "Delete draft" button before you leave the page.
Auto save drafts
While you are typing a message, every X seconds (currently 60), a draft of your message will be saved. If you reload the page later, your message will be automatically re-shown. Drafts will be automatically pruned over time. Each draft is associated with a particular piece of content, such as a thread, forum, or conversations. A draft reply that you start working on in thread 1 will not be shown to thread 2 and so forth.