Gotta say that personally, I am so anal about changing my own oil, I just can't see how one leaves the old gasket on the mating surface.
Just something that seems to be so bizarre to happen. Though I've heard of it happening before.
I have however... experianced another dealership's error (and my error for deciding to bring it in to them) when I asked them to adjust the valves on my past DL650. Asked them to go ahead and change the oil as well..
Trailered it in, then trailered it back out and brought it home. Started it up and turned my back to it momentarily. When I turned to look at it again, it was spewing oil all over the place from a oil filter that was not even close to tight at all.
No joy from bitching at the owner of the dealer afterwards. They just don't care.