The funniest thing that I have seen, happened about 20 years ago, when I was on a dog grooming course. One little dog a poodle, was being held on a lead and waiting to be clipped, when he started humping his lead. Now if he hadn't gone 'all the way' I would have said that it was anxiety that caused the behaviour, but he really did seem to be getting into the swing of things, and ended up by finishing it all off as well. He wasn't interested in the person holding the lead, just the lead.