Automatically wraps Bower packages as gems for the Rails asset pipeline, but duplicate JavaScript copies can conflict when mixed with other package sources.

Rails 3 requires rspec and rspec-rails 2, with renamed namespaces, changed requires, and macro extensions that may break during migration.

beginning_of_day and end_of_day on Date and DateTime ignore Rails 2 time zones, so database queries can hit the wrong records.

Rails has solid built-in protection, but some default behaviors remain risky in Rails 3 and parts of Rails 4. Security gaps can still leave apps exposed.

NewRelic monitoring can be restricted to selected Rails hosts by checking the current machine name before enabling monitor_mode.

Namespaced controllers and nested resources can align routes, controller classes, and view paths in Rails 2 and 3, reducing controller-directory clutter.

Spring can hide code changes while debugging Rails commands because it reuses a cached application process. Disabling it avoids stale behavior in rails and rake runs.

Bundled commands can fail on Ruby 1.8.7 and Rails 2.3 with undefined method 'name' for Array because rubygems-bundler or Bundler no longer works reliably on older setups.

Default Rails generators create different combinations of models, controllers, migrations, routes, views, and tests, helping choose the right generator from the command line.

Rails can report a missing mysql2 gem even when it is bundled if database.yml still uses the mysql adapter; older Rails versions need mysql2 0.2.x.

Setting up Rails on Ubuntu needs Ruby, Bundler, and the right development packages; repository RubyGems often causes problems, and project gems should be installed with bundle install.

Routing a dotted path can break Rails format detection, leaving format as html despite an XML constraint. Matching the route without the extension preserves the intended xml format.

Rails 3.1 lets a single change migration run forward and backward automatically, while irreversible updates are skipped during rollback.

Valid RSS output in Rails needs strict XML formatting and proper feed metadata; Atom builder can be a safer alternative for syndication feeds.

Dependency licenses can break closed-source projects when unapproved OSS terms slip in. License Finder validates all dependencies during test runs and flags restricted licenses early.

Rails commands can start Spring unexpectedly, and DISABLE_SPRING disables it only for non-explicit invocations. Different launch paths affect console and rake behavior.

Routing error in Rails 3.2.22.1 can appear when concerns is passed as an option to resources, producing broken routes and failed controller specs.

Rails 4 exception emails from ExceptionNotification may lose gem and library lines because the default backtrace cleaner filters them out.

Rails console fails after upgrading Ruby because spring tries to load a missing irb file. Updating binding_of_caller to 0.8.0 restores console startup.

Timezone abbreviations are ambiguous and often unsupported in Rails time handling; ActiveSupport zone names and offsets avoid wrong parsing, DST mistakes, and misleading displays.

Sidekiq::Web can create duplicate session cookies when Rails uses a wildcard cookie domain, breaking logout and other session-dependent actions.

Large routes.rb files become hard to manage as Rails apps grow. Splitting routes into small partial files keeps routing organized.

Run several MySQL releases side by side in one Linux account for testing or Rails development, using isolated sandboxes on separate ports.