"can't activate rails", "already activated rails", and "no such file to load -- breakpoint"

I've already asked about this on http://weblog.rubyonrails.org/2007/9/30/rails-2-0-0-preview-release
but am bringing my troubles here as Jeremy Kemper requested today.

After upgrading my gem rails to the 2.0 preview release (1.2.4.7794
was the first one I got) I tried creating a new project and installing
the acts_as_attachment plugin. Then I ran rake test:plugins to make
sure everything is compatible before proceeding with my work and I got
errors like this:

  rubygems.rb:246:in `activate': can't activate rails (= 1.2.4),
already activated rails-1.2.4.7794] (Gem::Exception)

A similar error occurs with acts_as_paranoid, though it complains
about one of the "active" gems rather than the rails gem itself.

Today I freshened my gems and installed 1.2.5.7919. I repeated the
steps above and got an equivalent error:
  ... can't activate rails (= 1.2.5), already activated
rails-1.2.5.7919]

I'd already tried freezing rails in various ways and I've messed with
RAILS_GEM_VERSION, both without success.

Today I tried gem cleanup rails, which left me with just 1.2.5.7919
instead of the six or seven versions I had previously. I created a new
project etc., as above, and when I ran rake test\plugins this time it
failed slightly differently:

  `gem_original_require': no such file to load -- breakpoint
(MissingSourceFile)
       from [...]/ruby/lib/ruby/site_ruby/1.8/rubygems/
custom_require.rb:27:in `require'

Can anyone offer any help with this?

Thanks,

Sven

The same issue occurs with the tests for attachment_fu and the 2.0
preview release. It works fine with 1.2.5. I don't know whether the
problem is rubygems not distinguishing between, say, 1.2.5 ans
1.2.5.7919, or whether there's a problem with the plugins'
dependencies. Am I really the only person trying test:plugins against
the 2.0 preview release?

-Sven

Sven, I just ran into the same issue. For me, it was an rmagick
problem caused by an upgrade to my OS. I had to uninstall the gem and
reinstall it and then everything worked fine. That may not help you
specifically, but I thought it was worth noting that it didn't have
anything to do with a plugin for me.