Any known problems with Ruby/Rails in Lion -> Mountain Lion upgrade?
I'm not sure about Ruby or Rails development specifically, but here's some things I had to do after upgrading to 10.8. See also this question and this blog post.


I had to install the command line tools package from Xcode's preferences. Xcode no longer includes separate binaries of the LLVM compiler or make or utilities like SetFile by default.
X11.app is no longer installed by default. Some Homebrew formulas required installing XQuartz.
Configuration files in /etc/apache2/ were removed or overwritten. See this question.
ImageMagick didn't work until I uninstalled and installed it again with Homebrew.