I hate this. I was not a frequent traveler until this year and ordered room service for the very first time. I had paid by credit card so the slip just said "tip", not "additional", so I left 20%. Then after they had left I reviewed the charge and saw that there was already gratuity on there. I was ticked!

Maybe there are some cheap people out there, but when there is mandatory gratuity, at least from me the server usually loses out on a few percent extra.