Frozen ActiveRecord instances raise can't modify frozen hash when attributes change; dup usually creates an unfrozen shallow copy, though Rails 2.3.x has a bug.

Rails form fields can clash when the same form is rendered multiple times; a prefix keeps field IDs unique without changing submitted names.

Bundler environment variables can shadow a Rails test app when integration-testing CLI tools with Aruba and Cucumber. Clearing the Bundler env and disabling Spring keeps commands isolated.

Virtual date attributes in forms arrive as strings, so Rails does not auto-cast them like database columns. A custom accessor can coerce them to Date objects or nil.

Virtual integer attributes in Rails can lose form selections and type casting on validation roundtrips; an auto-coerced accessor keeps values as integers or nil.

Rails submit buttons with :disable_with can stay disabled after back navigation, leaving forms unusable until they are re-enabled on page unload.

Rails can fail to boot after a Bundler upgrade when the new YAML engine cannot handle symbol values in localization files.

Rack can reject large nested forms by miscounting input fields against its parameter limit, causing low-level Ruby errors or Rails error pages.

update_all can ignore conditions when :order and :limit are present, causing unintended bulk updates in Rails 3.

Background job processing for Rails can avoid Redis infrastructure and managed Redis costs while staying compatible with ActiveJob through a Postgres-backed worker gem.

ActiveSupport::Memoizable is being removed and has awkward edge cases; Memoizer provides straightforward method caching across Rails versions.

Capybara 2.0 breaks compatibility with 1.x and drops Ruby 1.8 support. It changes Rails integration test conventions, element matching, and RSpec syntax.

Rails treats html_safe strings as already escaped, which can block escaping when safe HTML must be passed through JSON or shown as text. Converting to a plain String restores escaping.

Run an older Bundler release for compatibility when Rails or another project is pinned to a specific gem version.

Capybara can lose the HTTPS state when specs cross between secure and insecure Rails actions, causing request URLs to fall back to the wrong protocol.

Distributing array items into visual columns is awkward with in_groups and in_groups_of; in_columns returns wrapped column arrays in pure Ruby and Rails.

Rails hashes can be turned into URL query strings with Hash#to_query; browser-side nested objects can be serialized with jQuery $.param.

Rails asset paths depend on a leading slash when using javascript_include_tag or stylesheet_link_tag; files in /assets also need to be added to the precompile list.

Native file inputs can accept several uploads at once with multiple, but server parsing usually needs array-style names and custom Rails adaptation.

Custom .irbrc files can make irb and the Rails console more useful with extra methods, nicer printing, editor integration, and app-specific prompts.

Rough JavaScript equivalent of Rails simple_format for turning plain text into HTML paragraphs and line breaks. Whitespace preservation is not included.

Capistrano deployment warnings can appear when Rails asset pipeline apps lack empty public subdirectories. Placeholder files like .gitkeep suppress the missing-path errors.

ActiveRecord::Base#becomes leaves changed attribute tracking inconsistent, which can break attribute_was checks after casting models with defaults. ActiveType.cast is often the safer alternative.

Ruby Pathname objects can build file paths with :/ or join, making path concatenation in Rails more concise and readable.