Hydration Mismatch Error - Server and Client Render Different Content

1. Using client-only features (browser APIs) during server render
2. Rendering different content based on window/document being defined
3. Timestamps or random data rendered server-side changing on client
4. Conditional rendering based on isClient flag without useEffect
5. CSS-in-JS not being applied on server during build

Move browser code to useEffect: useEffect(() => { setMount(true); }, [])

Use dynamic imports for browser-only components: dynamic(() => import('@/components/ClientComponent'), { ssr: false })

Don't use Date.now(), Math.random() during render - do it in useEffect

Check if typeof window === 'undefined' before using browser APIs