Hmmm. Quick look makes me think it's related to categories that are subcategories... Noticed as well it shows on unregistered users... will need to think of a way of stopping that happening.
Partly for my note I noticed that this happens because their as an ol/li combination for subforums when using forumdisplay. The first + button is linked to the identity of the parent forum (in the above example forum ID 69) whereas the second relates to the actual subforum. Need to look at figuring this one out...