Yep. It was because slugbuilder didn't lock buildpacks to a known working version when you ran a `docker build`. It always pulled master, which may or may not work. We fixed that up, and we're in sync with Heroku's buildpack version that's running in prod now: https://github.com/deis/slugbuilder/blob/deis/builder/instal...
I opened an issue request with Deis a while back for the same problem and they reported it being an upstream issue with slugbuilder.
Edit: Heh, apparently we already talked about this https://github.com/deis/deis/issues/1016