Thrillist has put together a list of shady strategies SOME bartenders tend to practice. I emphasis some because, well, I have worked behind the bar in my lifetime and I know most bartenders actually do not use these mean methods of "stealing" from their bar patrons.
I will warn you to be prepared, because, whereas most bartenders do not conduct in such a shady manner, I have seen it done and have had friends tell me their experience with at least two or three tactics on this list. So, you may get a little bitter (no pun intended, whiskey drinkers) by the end of this article. Cheers.