A kohen who isn't wearing all of the priestly garments cannot do the service. And he has to wear them well - but if they are too long or too short or too worn out, that would still be acceptable. But not if we doubled up or left off one of them (4 garments for a regular kohen, 8 for the kohen gadol). What about a bandage (we'll see tomorrow that it depends). Or dirty. All of those factors would invalidate the service. Also, the fabric of the garments have very specific criteria as well - in terms of it being fine linen and the way the threads are made.