I've listened to several recordings of Chopin's Nocturne in C# minor, op. posth., and 90% of people double the tempo at the end of the middle section, when the meter changes from 4/4 to 2/4 and then 3/4. Why do they do this? There is nothing indicating this change. Perhaps if the meter changed to 2/2 and 3/2, THEN I might understand (even then, it'd be ambiguous).