This is going to sound really stupid, but first and foremost, turn the tablet on it's side and blow into the USB connector to make sure that there's nothing stuck in it. In my endeavors of creating the many hundreds of USB OTGs that are now circulating the community I've had small metal particles drift into there before and cause the OTG pin to short as Benson described. Not saying that's the cause, but definitely the easiest thing to check and possibly solve the problem (especially if it's still happening after reboot). Also double check to make sure that you haven't knocked the metal pins in the USB connector around, as it is physically possible to shift them accidentally while removing a USB plug on a funny angle (a tiny flat head screw driver will usually let you just tap the offending pin back into a straight position).
The message can also be caused by software forcing a rescan of the USB bus (possibly Canola does this from time to time?) but there are many possible causes for this and you simply have to troubleshoot by removing/closing software to see what's actually causing it.

Thanks,
-Rob