Although it is not a problem in these classes where you are very specific and current about gem and ruby versions and configuration (Thanks!), when I try to use older 3.x books and learning resources I have run into several frustrating conflicts using incompatible versions of ruby, rails and macosx.
Are there any online resources that help describe how to resolve ruby/gem compatibility issues, like listing which ruby versions work with which gems, which ruby/rails versions to avoid using and why(eg. security), or describing the cautions on the gotchas like needing to add back gcc with command line tools on newer OSX systems? Its has been a slow and frustrating hunting game for me on stack overflow, and other similar sites to resolve these issues when building learning apps. gem and version conflicts and finding solutions waste a lot of productive learning and coding time for beginners.
It would be helpful to know that when a book specifies using a rails version like 3.2.11, you should only use these ruby versions or gem versions with it, and which commonly used gems are not compatible with specific ruby or rails versions in a list format. Bundler tries to help you resolve conflicts, but online content explaining what works together and why not would be informative teaching material and save time.
On a related note, as learners try building new apps with rails4.x and ruby 2.x, it would be nice to have transition help, listing which commonly used gems are incompatible and providing a checklist (or scripts) to help to transition a 3.2.x app to rails 4 app, esp when using 3rd party gems. Is there a place that you can recommend where we could find this information online? (related to the helpful post from slothbear) I would love to see a ruby2/rails4 version of each thoughtbot course app as a repository with learning notes. The only material I found about transitioning was on Railscasts #415.
I know that it would be hard to maintain as current if it exists, but an online ruby/os/gem compatibility list with recommended solutions for conflicts would be much easier to use than a variety of inconsistent answers from various forum/chat sources. I think that bundler is very useful for what it does, as is stack overflow. From stack overflow, it looks like many of us are having the same problems and headaches. We could avoid conflicts if we had a watch list with explanations, and knew ahead of time what to avoid and how to transition when building rails apps. I could help maintain a list like this, but I don’t have the knowledge or experience to get started. 
Thanks!