We have recently recieved a cat that was injured by it's own collar. Some collars are not as 'safe' as they appear.
One of the main culprits are the elasticated collars that class themselves as 'safety collars' because they stretch. What is often the case is that because the collar is too loose, the cat gets one leg through, as illustrated in this stock photo, and then cannot get the leg back. The collar remains in this position getting tighter and rubbing until a lesion forms. As you can see from the second photo of Melody, a skin graft is taken which is then used to cover the lesion in the cats armpit - this is a dramatic surgery that does not always take and can take 10 plus days to heal. If your cat must wear a collar, please ensure it is the quick release click type and not a buckle or elastic type.