Capybara feature specs do not reliably catch server exceptions in another thread; request specs or visible UI assertions fit this user-level behavior better.

validates_numericality_of converts values to Float or integer, so high-precision BigDecimals can round and fail comparisons unexpectedly.

Uploaded HTML or SVG can execute JavaScript and steal session cookies; safer uploads rely on extension allowlists, strict CSP, attachment disposition, or octet-stream delivery.

Rails default ETags often never repeat because rotating CSRF tokens and CSP nonces change response bytes, blocking cache hits and 304 responses.

ActiveRecord::Relation#merge can overwrite conditions on the same column and return too many records; where, subselects, and, or SQL snippets preserve the intersection.

Capybara matchers and finders can take a Ruby block for extra DOM conditions when built-in options are not enough. Script calls inside filter blocks need a temporary wait time to avoid Selenium timeouts.

Prioritizing refactoring and upgrade work is harder in larger projects; a maintained technical-debt summary helps compare effort, customer value, and developer value.

fresh_when can combine multiple records into one ETag so associated data keeps cached pages invalidated when any rendered record changes.

Strict loading in index actions catches N+1 queries when views touch unpreloaded associations, causing test failures instead of hidden database hits.

Rails partials support locals, yield, layouts, collections, empty states, controller rendering, and cached collection rendering for reusable view markup.

HTTP header values are limited to low-ASCII for safe transport; non-ASCII data such as Umlauts or emojis can be carried by JSON-escaping characters above 127.

Icons and other inline blocks can be centered against capital letters instead of the text baseline. vertical-align with font metrics or 1lh gives a reliable result.

Frozen time in tests can make suites flaky when state leaks between examples. Resetting Timecop after each test or using block form prevents order-dependent failures.

Imported ESM functions are hard to spy on in Jasmine; several workarounds keep call counts testable, including wrapper objects, dependency injection, and test runners with module mocking.

Kernel#open and OpenURI#open_uri can still trigger real network requests in tests because VCR and WebMock do not mock them.

<sub> and <sup> often break line height in email layouts; setting line-height: 0 with mso-line-height-rule: exactly keeps superscripts and subscripts consistent across clients.

SVG icons can be sized and colored like text without image tags, icon fonts, or extra server work by using CSS masking.

Efficient ways to create DOM test fixtures for Jasmine, from HTML parsing and selector helpers to innerHTML, with readable structure and direct element references.

DOM-based Jasmine specs can leak leftover elements into later tests; using a dedicated fixture container cleared after each run keeps test state isolated.

Time values in ETag generation can lose millisecond precision, causing stale cache hits and flaky tests. Convert them to strings or numbers before combining cache keys.

Flaky specs often come from brittle matchers, nondeterministic record limits, and UI components that load or animate asynchronously.

git restore reverses staged, unstaged, deleted, or conflicted changes with clearer intent than git checkout, and can restore files from other revisions.

Removing local Git modifications is useful when work needs to be undone, parked temporarily, or cleaned from tracked and untracked files without affecting remote history.

Regex find-and-replace in RubyMine uses capture groups and backreferences to rewrite quoted text, URLs, and old RSpec syntax safely.