For example say you need to patch Hibernate or ActiveRecord with a fix that hasn't been released yet. Then you want to roll out that fixed/forked version to your team, then once the new release comes out with the patch, you want to switch the team back to the main release. Of course with a team you want to make this as painless as possible, ideally, without them having to do anything.
In Java (especially using Maven) I find it relatively simple to fork a jar or plugin with my own patches and have the team start using that transparently, without them having to know or do anything beyond a CVS update. Then when a new release of say Hibernate comes out with the fix in question I just update the dependency in project.xml and the whole team automatically switches over to using the standard distribution again. Many team members may not ever know or notice the switch even happened.
Now, with Ruby I'm not even sure how I would go about transparently getting the team to use a forked version of ActiveRecord without having to go around and ask each of them to do something. Or perhaps there's an easy way to do this with Ruby that I'm not familiar with?