The worst thing I've seen happen at a drive-thru is a couple of gangsta types in an old Cadillac get mad at the help and attempt to throw the drink they were given back in the window cashier's face. She had closed the window and their effort was to no avail; soda and ice bounced harmlessly off the window, they then sped off.
I thought it could have gotten worse, the cashier was okay when I asked her, just a bit shaken. Apparently they had cussed her out and she didn't know if they might have been dangerous people. Fortunately, it turned out all right.