Military rifles wrapped in parafin wax and brown paper can come out factor new.
Handling and moving stuff about is asking for a knock or too. Oil from fingers will burn into bluing. Doesn't take much. Wood can get over dry and so go brittle.
Gunshop racks are terrible.
Worst culprits are those who "restore/fix" with no knowledge of what they are doing or how to do it correctly. "I gave it a clean" can be heartbreaking.