well if your coils are dead, the spider separated from the cone, the coils scrape... umm pretty much if you think/know your sub is failing or failed //content.invisioncic.com/y282845/emoticons/confused.gif.e820e0216602db4765798ac39d28caa9.gif
when your Voice coil launches through the front of your cone, or your cone shreds / explodes.
when you seperate VC from former, rip spiders, melt / burn coil / tear or pop tinsels. If your sub plays fine and does not scratch during excursion or bttom out easily then you dont need a recone.