Don't leave clip-on tuners on your headstock after you've tuned yur guitar. The softeners in the rubber pads will ruin the finsh if they're left on for long periods. Tune it, then take it off.
I have a friend who lost part of his Martin decal to the pads on his Snark.
This also goes for leaving a vinyl backed strap on your guitar while it's not being played, and don't leave your guitar, mandolin . . . on a Naugahyde chair or couch.
Besides being a hazard to the finish, tuners, capos, cigarettes and unclipped strings sticking out from a headstock is just plain ugly.