IRB and the Rails console store the previous command result in _, making it easy to reuse a forgotten value without rerunning the expression.

ImageMagick can remove transparent borders and fit images into a fixed box without distortion. -trim, -resize, and -extent preserve aspect ratio and center the result.

hash_including cannot match nested hashes reliably; use nested matchers such as include or another hash_including for partial hash checks.

Changing CSS properties can trigger different amounts of browser work, affecting performance when styles are updated.

ActiveRecord::RecordNotFound carries model and id, making it possible to report missing records without exposing internal query details.

Rails can nest flat JSON request bodies under a root key so strong parameters work like form submissions and stray keys such as format stop interfering.

The Snom 300 can tie headset and ringtone volume together when the ringer output setting is misconfigured. Selecting only one output device separates the volumes.

Set up an Ubuntu computer as a gateway to share one internet connection over a local network, including WLAN or tethering sources.

Select2 can be styled to match Bootstrap 4, using a Bootstrap 4 theme fork and matching Sass imports for consistent dropdown controls.

Capistrano rollback hooks let custom deployment tasks undo side effects after a failed release, including Slack notifications and other deployment-specific actions.

Compile Webpacker assets once in parallel test runs and skip recompilation when files are unchanged, avoiding duplicate work and Capybara timeouts.

Detects whether a visible page element is covered by another layer, such as a modal overlay or higher z-index element, using a browser hit test.

CarrierWave can temporarily keep three copies of an uploaded file, and large uploads may waste time and disk space. Configuring moves instead of copies reduces the duplication.

Struct.new can define lightweight Ruby classes with attributes, comparison, and keyword initialization for data objects that behave more like records than hashes.

Rails caching can be enabled for a single RSpec example without changing global test settings. File and memory stores let cache behavior be verified in isolated tests, including parallel runs.

Conditional Rails validations with if: are not covered by Shoulda Matchers; test the model’s behavior by stubbing the predicate and checking presence only when it applies.

Serve raw GitHub files with correct Content-Type headers for CDN-style testing or prototyping, but without any uptime guarantee.

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

Manipulating PostgreSQL array columns can be awkward: values can be appended, removed, or replaced, with special handling for multi-value substitutions.

ChromeDriver click targeting can hit the wrong point on an element, causing intercepted or non-clickable errors when overlays or nearby elements overlap.

Rails 5 model classes inherit from ApplicationRecord, making it the shared home for logic used by all application models instead of patching ActiveRecord::Base.