Currently I found an issue where jquery-ui was not being found for precompile.
Making me wander and loose some time, after I figured that I updated all my gems, making this one update to 3.something...
gem 'jquery-rails', '2.2.1'
If it was not for git, I would have spent some more time finding this out.
Just lock your gems, and update them whenever new changes or security issues arise with one of them. Modify the gem version on the gemfile and;
bundle update gem_name