Beethoven's Quartet in C# minor, Op. 131, stretched the format of the string quartet so far it broke. Instead of being in four separate movements, it's in seven joined movements that create one long arch of sound. Here's the format, and boy, is it strange!
Concerning this last set of string quartets, the first violinist of the quartet that premiered all of the Beethoven quartets wrote in Beethoven's conversation book: "The players are having difficulty understanding these latest quartets."
Beethoven answered, "Oh, I didn't write these quartets for you! I wrote these for a later generation."