We pretty much always see this at the end of era's. Musical genres/styles are never created in one moment, just foreshadowed (in my opinion as a music historian). My favorite example of this is Beethoven and Schubert's use of uncommon key areas in already existing forms.

Explanation on how this anticipates coming practice and diverges from the then standard harmonic practice:

In the classical era we mostly see an emphasis on the following keys (I'm going to use sonata forms here): If we're in major we'll see a lot of I (the tonic key in major) and a lot of V (the dominant key in major and minor, although sometimes it's little V, or the minor dominant). We also see very clear sections where one key is present. A sketch of a sonata form looks as follows:

First theme (in I, the tonic key), 2nd theme (in V, the dominant key) , Development section (lots of different keys for a bit), First theme and 2nd theme again (only in I this time).

However, Beethoven and Schubert did some crazy stuff. Schubert has a sonata form where the following keys are presented before the development : C Major (I), Eb Major (bIII), f# minor (#IV) and A Major (Major vi, but not flat) (This is his String Quintet in C Major) . These key changes are often sudden and unexpected. They foreshadow the coming romantic era's tendency to change keys on a whim.