One of the nicest coins in my collection I bought from a chap in France who says that, at the time (in 1909) his grandad went to the bank and changed up for various denominations, then wrapped a few mint examples in silk(?) pouches and tucked them away. The penny I bought from him was one of those hidden away since that day, and it's a true stunner.
I guess even coins in collections are prone being moved around, touched, breathed on etc..