We recently had new double-hung windows installed in our row house built in 1900. (The previous windows appear to have been done some time in the 1980's, so nowhere near original.)
We relied upon a well-reviewed-on-Angie's-List company to do the install. When the top sash is pushed all the way up and the bottom sash all the way down, there's a roughly 1/4" mismatch in the vertical alignment of the sashes (seemingly consistent across the width of the window). It's possible to align the sash and latch them, of course, but then there's a 1/4" gap at the top or bottom or distributed across the two, depending upon whether you push the pair up or down.
The company's proposed solution is to tack a 1/4" strip of wood to the sill under the lower sash. I think this looks preposterous. They're the ones that measured the opening, and it seems they made a mistake.
I could believe that there's a small fudge factor maintained for thermal expansion, but this is the middle of summer. Any gap we see now would only get bigger as it cools.
So, I'd love to know if this (tacking a strip to the sill) is common practice or if I should argue that the company should provide correctly sized windows. I could also imagine that difference could be made up by adding a strip at the top of the frame instead of the bottom where it wouldn't be visible from the outside due to the lip up there.