Smoke testing is a broad, shallow pass on the major features of a build — it verifies the app launches, critical flows work, and the build is stable enough for further testing. Sanity testing is narrow and focused: after a bug fix or small change, you verify that specific area works correctly without running the full suite.

Start with the happy path (valid credentials), then: invalid password, non-existent email, empty fields, SQL injection attempts (' OR '1'='1), case sensitivity on email vs. password, fields exceeding max length, special characters, account lockout after N failed attempts, concurrent login from two devices, and "forgot password" flow. On mobile: autocomplete, clipboard paste, keyboard behavior.

XPath is an expression language for navigating XML/HTML tree structures. Use it when: elements lack reliable IDs or CSS attributes, you need to traverse parent-to-child or sibling relationships (../preceding-sibling), or locate elements by text content (//button[text()='Submit']). CSS selectors are faster and more readable for stable attributes — prefer them unless you need the traversal power of XPath.

WCAG 2.1 Level AA is the accessibility standard most laws reference (ADA, EN 301 549, AODA). It organizes requirements into four principles — Perceivable (text alternatives, captions, sufficient contrast ≥4.5:1), Operable (keyboard-accessible, no seizure-inducing content), Understandable (readable text, error identification), and Robust (compatible with assistive tech like screen readers). QA engineers are responsible for catching violations before legal exposure occurs.

A race condition occurs when two concurrent operations compete for the same shared resource and the outcome depends on their execution order. Classic examples: applying a discount coupon twice simultaneously, double-booking an appointment slot, or crediting a bank account from two concurrent requests. To test: use tools like Apache JMeter or write parallel requests in code to hit the endpoint simultaneously. Check the database state afterward — if you got two credits or two coupon applications, the race condition is confirmed.