If you try to drag a topic to the first position it sometimes fails to work, even though it looks like it has worked (i.e. the topic switch places), when you do anything else (e.g. turn off editing) you find the topic is back in the place it began.
You can tell if it has worked because the little spinner icon appears as it is working. When it's only pretending to work, you don't get this. (If you've got developer tools in your browser then you can see the REST call being fired as well when it's working properly).
This seems to have something to do with the drag target. If you go onto Topic 0, then you get feedback as if dropping would succeed, and when you release it looks like it has worked. The workaround is to drop it lower down the screen.
I've tested and seen this behaviour in Firefox, Chrome, IE8 and Safari (though Safari might have other issues, it seemed the least consistent).