I don't see this very often but I do see it in 1 of Chopin's nocturnes. In particular his F major nocturne op. 15 no. 1
It switches back and forth measure by measure between 3/4 and 9/8, well at least in some editions it does.
A lot of editions just have a 3/4 time signature throughout the nocturne which makes more sense to me. After all 9/8 is just a compound version of 3/4. It is like 3/4 with triplets essentially. But why then would some editions have this measure by measure switch between 3/4 and 9/8 if they are essentially the same but expressed differently? I don't see any reason to have those 2 time signatures side by side. Now if it were 3/4 + 2/4 that would be a different story but here the 2 time signatures are essentially the same.
So why even write 2 time signatures side by side if either time signature would fit equally well to the entire nocturne and 2 time signatures side by side means a measure by measure switch?