Usually this does not matter as your execution is using the latest versions of your gems. However if you end up downgrading your gems your project might accidentally use wrong dependencies.
Then you should clean your gemset using bundler:
$ bundle clean --force
As you revert back to e.g. Rails 3.2.8, your activesupport 3.2.11 will not be updated, and your specs will still fail (unless you strictly use
bundle exec <command>). After running
bundle clean --force, if your gemset has been correctly set, you don't need
bundle exec and all will work as expected.