I'm happy to lend my guitar (not an expensive one but it sounds good) to anyone who may have either forgotten or hadn't brought theirs for whatever reason but I remember lending it to someone once and when I got it back I couldn't play it - it was like the chap had spread evostik (contact adhesive) all over the neck and the strings.

It might well have been that he had the sort of skin/sweat that had this effect or he'd been eating a finger buffet beforehand and not washed his hands. I now keep a duster with polish on it and a cloth with some degreasing spray in the case just in case I meet someone with the same problem.

Cheers
Mark