Problem: Gemfile.lock was generated on Windows machine (mingw), but on the OS X machine, the mingw gems are removed from the lockfile (and the platform "unknown" is added).
Bundler Version: All developers on are on bundler 1.0.13
Ruby Version: 1.8.7-p174 (OS X) and 1.8.7-p330 (Windows)
RVM Version: 1.2.4 (OS X), no RVM on Windows
Bundle Config: no settings