Fix Hydration Errors in Next.js

Common Causes of Hydration Errors:

Browser/Environment Issues:

Browser extensions injecting attributes (password managers, ad blockers, accessibility tools)
Browser auto-fill adding attributes to form elements
Third-party scripts modifying the DOM before React hydrates
Different timezones causing date/time mismatches between server and client

Code Issues:

Using browser-only APIs during SSR (window, localStorage, document)
Random values or Math.random() in render (server generates different value than client)
Date objects using new Date() without consistent formatting
Using useEffect or useState that modifies content on initial render
Inconsistent whitespace in template literals (extra spaces, newlines)
Conditional rendering based on client-only values (window.innerWidth, user agent detection)

Data Issues:

Server and client fetching different data or at different times
Missing key props in lists causing reordering issues
Using non-serializable data in server components (functions, symbols)
Inconsistent data formatting between server and client (dates, numbers, locales)

CSS/Styling Issues:

CSS-in-JS libraries that generate different class names on server vs client
CSS modules with inconsistent hash generation
Media queries that apply different styles on server vs client

Component Issues:

Nesting block elements inside inline elements

(<div> inside <p>)

Invalid HTML structure (SSR might render differently than browser parses)
Third-party components that aren't SSR-compatible
Portals rendering in different locations

Best Practice: Always check if code relies on client-only APIs and wrap them in useEffect or use 'use client' directive appropriately.