Ten years into marriage, my darling man and I hit a milestone today: we managed to fail to accomplish installation of a window air conditioner without grumping or snapping at each other.
...the failure part is because apparently we need a 1/2" wood shim to bring the brackets up to a level where they'll clip into the bottom of the unit. This is not mentioned in the directions, as apparently our windowsills do not meet the ideal windowsill installation that it was designed for. So, drawing inspiration from the "if you have storm windows" section, there will be a run to the hardware store, a shim installed to raise the brackets to the height that they'll meet the bottom of the a/c, and things should be good.
The windows are, as far as I can tell, about 25 years old. I shouldn't be surprised that there have been design changes to the sills. Ah, well!
Note that I did not cuss at my darling man, and we didn't yell at each other. Actual air conditioning unit, included manual, and cats who were trying to help not included in this accomplishment.