1.9.1->1.9.2 had breaking changes (most obviously require_relative) and 1.9.2 had performance issues in requiring files. I haven't run into any issues with Ruby 1.9.3, but it's only 1.5 years old, and even 1.9.3 was not 100% backwards compatible.

I understand that this all seems like eternity if you are actively developing a Ruby app, but for everything that is in maintenance mode, 1.8.7 has been a very sane choice until now. I had certainly planned to use it another couple years if possible. :/