It does it because It goes to a entry that isnt real. So it goes to a blank page.
Yeah, you can see the "st=-20" part of that. Its trying to start displaying the thread from the negative twentieth post, which just isn't possible. It isn't really much of a dangerous bug, because it won't come up unless someone puts it there.
See where it says st in the URL? That goes straight into the query where it says "LIMIT". If you put -20, it tries to limit it to -20 posts, therefore none are selected.
Click on the link and click on queries.
The st variable tells it what post in the topic to start from. It has nothing to do with a limit. Try putting in 9999, it won't work... it'll correct itself to zero.
This isn't a bug at all. If you want to view threads starting at -20, thats abosolutey fine with me. Knock yourself out.