Most people these days use an electronic tuner on the headstock of their instrument. While they are very handy little gadgets, they do have one major flaw - the rubber feet on the tuner can react with the finish on many instruments, leaving a dark, permanent stain. The reaction between the rubber and the finish goes deeper than the surface, so you can't just wipe it off or even buff it out.
You can easily avoid this problem by simply removing the tuner from your instrument when you finish playing. This might seem obvious, but a lot of folks figure, “Well, I’m going to use the tuner as soon as I pick up my instrument to play it again, so I’ll leave the tuner on.” While we all appreciate that you are diligent about playing in tune, we suggest you take a few seconds to remove the tuner and store it somewhere that's not in contact with the finish of your much-loved instrument.