If you have a guitar or a bass with a nitrocellulose lacquer finish, be careful with your strap. Certain leathers and synthetics can react with the nitro and may discolor it. In particular, don’t leave the strap on the instrument when you’re not playing it.
Likewise, be careful of certain guitar stand and guitar hanger pads; the materials these cushions are made from may also have an adverse affect on a nitro finish.