If you've just installed rails edge and run the tests, they may fail with: NoMethodError: undefined method `expects' for ... etc etc over and over again (along with a few other errors).
Install the mocha gem via rubygems to get rid of a lot of these errors.
It was the line NoMethodError: undefined method `stub' for ... that clued me in.
It seems that rails requires a short list of gems/packages (beyond rails itself) simply to run its own tests... yet there is no rake gem:install task available to help you figure out which ones... and they aren't in the "contributing to rails" guide. I'll be submitting a patch shortly...
Following on from this I got:
MissingSourceFile: no such file to load -- openssl
and farther down the list:
LoadError: no such file to load -- net/https
Unfortunately, these errors occur when Ruby hasn't been installed with openssl-support. If you're running ubuntu, you can just run apt-get install libopenssl-ruby.