To be honest, I don't remember the technical specifics...too much time has passed. It wasn't so much that a breaking feature change required rewriting code as it was that a single failure propagated. One component written in Scala on which my project depended was upgraded to 2.10, and a backwards-incompatible change in case classes broke the ability to serialize/deserialize objects between my project and the other component.

Upgrading my project to 2.10, several of the dependencies no longer compiled. I don't remember which ones specifically, but I believe one of them was a JSON library. We had to find libraries that fulfilled the same roles, then rewrite the code that used those libraries against the new APIs. It wasn't horrible in the sense of there being a nasty, hard-to-detect runtime bug, rather that it required basically redoing all of the "glue code" with respect to libraries.

This wasn't painful enough to scare me away from Scala in the future, though it does make me leery of using Scala libraries from anything but top-tier vendors. That is, I'd use a library from Twitter, FourSquare, TypeSafe, Akka, etc. but never again from anything not widely known.