# Frontend Debugging: React, Next.js & Browser DevTools

Hydration Errors: The Next.js Headache

Hydration errors are the most confusing Next.js bug. They happen when the server-rendered HTML does not match what React expects on the client.

Error: Hydration failed because the server rendered HTML
didn't match the client.

Common causes and fixes:

// BAD: Using browser-only APIs in server components
function Greeting() {
  // window is undefined on the server!
  const time = new Date().toLocaleTimeString();
  return <p>Current time: {time}</p>;
}