This commit: http://github.com/rails/rails/commit/fd0289f3ddf16ca08d6e9510dbf1a7e3ef4126df
bumped the required version of Rack to 1.0.1, but the actionpack-2.3.5 gemspec doesn't reflect that. The result is that installing 2.3.5 on a system with 2.3.2 installed (which grabs rack-1.0.0) fails when the server starts.
Patch here: https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/3558
Sorry to bug the list about a tiny (one character!) issue, but it didn't appear to be going anywhere on LH.
--Matt Jones