Error const_missing': uninitialized constant Rails::Plugin::Err (NameError)

I have installed some plug ins like will_paginate and
classic_pagination.
After this I have came across this error
/var/lib/gems/1.8/gems/activesupport-2.0.1/lib/active_support/
dependencies.rb:478:in `const_missing': uninitialized constant
Rails::Plugin::Err (NameError)