[chillfox]

Ruby 1.8.7 -> 1.9.0 caused a lot of problems in the sysadmin space that took years to get resolved.

[tyre]

triggered.

There was a rails upgrade around that time that was similarly painful, at least in the humongous rails app I was working in.

[andruby]

Rails 1 to 2 was a big one indeed. Rails 2 introduced restful routes iirc, which meant refactoring a lot of fat controllers.