Using self-hosted or purchased fonts with Webpack or Webpacker requires either bundling them from assets or packaging them locally for clean imports.

GROUP_CONCAT aggregates values per group into a comma-separated string, but truncation at group_concat_max_len can silently corrupt results unless the limit is raised.

Exchange can discard the plaintext part of multipart/alternative email, leaving only HTML in normal mail accounts and complicating debugging.

Upgrading to the mysql2 gem can require a deploy-time database.yml change; one adapter line can keep the same config working with both MySQL libraries.

Older Bundler versions can reject legacy Gemfile.lock files as corrupt when dependency metadata changes; adding missing gems or downgrading Bundler restores installs.

Limit Unicorn to one worker in development to keep debugging output manageable and prevent concurrent requests like ActionCable from flooding the console.

Binary file uploads need FormData so AJAX can send multipart payloads from file inputs without manual serialization; older browsers often need a plain form fallback.

Static assets on CloudFront can speed up page loads, but cache invalidation, gzip handling, SSL, fonts, and mixed-content rules need careful handling.

Ace editor adds syntax highlighting and auto-indenting for in-browser code editing, with a working brace setup for Ruby on Rails Webpacker and Webpack 2.

ONLY_FULL_GROUP_BY in MySQL 5.7 rejects queries that select non-grouped columns, preventing nondeterministic results and breaking some Rails scopes.

Store associated values as a single array-like field, editable in forms and restored safely after validation errors, while syncing to the underlying relation.

Cross-subdomain session cookies in Rails 3 need a shared cookie domain; local development can fail with CSRF token errors when browsers reject the cookie.

Passenger can fail with a misleading EOF error when the app spawner exits unexpectedly, often because a gem is missing. bundle install usually resolves it.

Mismatched Node.js versions can trigger Yarn integrity errors and unexpected node_modules changes, especially when dependencies are committed or .nvmrc is missing.

Public file downloads may miss Content-Disposition, so filename checks fail in Spreewald. Matching only Content-Type or testing the downloaded file avoids flaky download assertions.

Nested drag-and-drop for reorderable trees is hard when items can be leaves or containers; precise drop targets need custom JavaScript, CSS, and testing support.

\G in the MySQL shell switches query output to a vertical format, which is easier to read for wide rows or long text fields.

Private S3 files can be streamed with time-limited URLs instead of exposing a reusable bucket link. expiring_url and query string authentication help prevent easy redistribution.

RSpec examples tagged :vcr automatically record and replay HTTP cassettes, while untagged specs block network calls unless HTTP access is explicitly enabled.

rake middleware reveals the ordered Rails request stack, useful for adding, removing, or profiling middleware that runs on every request.

Savon mocks usually require an exact SOAP request body, but message: :any accepts any payload and returns the stubbed XML.

Rubygems version mismatches can break generated executables with undefined method activate_bin_path after downgrades. Reinstalling gem executables with gem pristine --all or reinstalling RubyGems and Bundler restores working binstubs.

Turn an Ubuntu PC into a simple internet gateway for another computer using a second network interface and connection sharing.