Error map
uncontrolled to controlled Warning: A component is changing an uncontrolled input to be controlled
React · layer 1, On your device
Centre: this error. First ring: errors it is linked with (12). Outer ring: what those are linked with. Colours show the layer of a request each error belongs to. Click any dot to re-centre the map on it; hover to follow its connections. The centre dot opens the error page.
Directly related (12)
TypeError: Cannot read properties of undefined (reading 'x')
You tried to access a property on a value that is `undefined` (or `null`) — usually data that hasn’t loaded yet or a wrong proper…
Flutter: setState() called after dispose(): _MyState#1a2b3 (lifecycle state: defunct, not mounted)
An async callback (network, timer, stream) finished after the widget was removed and then called setState.
Contact Form 7: There was an error trying to send your message. Please try again later.
The form submitted, but wp_mail() failed to send — the server can’t send mail or the mail settings are wrong.
Error: No QueryClient set, use QueryClientProvider to set one
TanStack Query hooks ran outside a QueryClientProvider (or a duplicate copy of the library is installed).
ERR_CACHE_MISS: Confirm Form Resubmission
You pressed Back/Refresh on a page that was the result of a form POST, so the browser asks before sending the form again.
Warning: Cannot update a component (`Parent`) while rendering a different component (`Child`)
A child component updated a parent’s state during its render instead of in an effect or event handler.
Warning: validateDOMNesting(...): <div> cannot appear as a descendant of <p>.
The JSX produces invalid HTML nesting, which also causes hydration mismatches in SSR.
Warning: Function components cannot be given refs. Attempts to access this ref will fail. Did you mean to use React.forwardRef()?
A ref was passed to a function component that doesn’t forward it.
Error: Minified React error #31; visit https://react.dev/errors/31 for the full message
A production React build threw an error whose message was shortened to a number.
Error: Maximum update depth exceeded. This can happen when a component repeatedly calls setState inside componentWillUpdate or componentDidUpdate.
A component keeps updating itself in an effect or lifecycle — usually a useEffect that sets state its own dependencies change.
Optimistic lock conflict: OptimisticLockException / StaleObjectStateException / version mismatch
Someone else changed the record after you read it, so your update — based on an old version — was rejected to avoid overwriting t…
API Error: Invalid state transition — operation not allowed in the resource’s current state
The request is valid, but the object is in a state where this action isn’t allowed — cancelling something already completed, refu…
Two steps away (35)
System.NullReferenceException: Object reference not set to an instance of an object
Code dereferenced a variable that is `null` — the .NET equivalent of Java’s NullPointerException.
TypeError: x is not a function
You called something as a function, but its value is not a function (it’s undefined, a string, an object…).
ReferenceError: x is not defined
Code refers to a variable or function name that doesn’t exist in the current scope.
HTTP 409 Conflict
The request conflicts with the current state of the resource — e.g. a duplicate or an edit based on an outdated version.
Error: Too many re-renders. React limits the number of renders to prevent an infinite loop.
A state update runs during render, which triggers another render, which updates state again — forever.
java.lang.IllegalStateException
A method was called when the object wasn’t in the right state for it.
PHP mail() returns false / emails not delivered
PHP’s mail() function failed or emails never arrive (spam or rejected).
SES MessageRejected: Email address is not verified. The following identities failed the check in region
SES refused to send because the From address/domain isn’t verified — or the account is still in the SES sandbox, where recipients…
WordPress: 404 Not Found on every page except the homepage (permalinks / .htaccess / Nginx rules)
Pretty permalinks aren’t being routed to index.php — .htaccess is missing/ignored (Apache) or Nginx lacks try_files.
RuntimeError: Working outside of application context.
Code used current_app, g, or a Flask extension (like SQLAlchemy) without an active Flask application context.
Adjacent JSX elements must be wrapped in an enclosing tag. Did you want a JSX fragment <>...</>?
A component returns two sibling JSX elements without a single parent.
Error: useNavigate() may be used only in the context of a <Router> component.
A React Router hook ran in a component that isn’t rendered inside a Router provider.
TypeError: Failed to fetch / NetworkError when attempting to fetch resource
The browser couldn’t complete the request at the network level — CORS, DNS, offline, blocked mixed content, or the server refused…
QuotaExceededError: Failed to execute 'setItem' on 'Storage': Setting the value exceeded the quota
localStorage/sessionStorage is full (about 5 MB per origin) or storage is disabled.
NotAllowedError: play() failed because the user didn't interact with the document first.
The browser blocked media autoplay with sound until the user interacts with the page.
Warning: Each child in a list should have a unique "key" prop
When rendering a list with `.map()`, each element needs a stable, unique `key` so React can track items between renders.
TypeError: Cannot assign to read only property 'x' of object
Code tried to modify a frozen object, a getter-only property, or immutable state.
Error: Element type is invalid: expected a string (for built-in components) or a class/function (for composite components) but got: undefined
JSX referenced a component that is undefined — almost always a default/named import mismatch.
Error: Hydration failed because the initial UI does not match what was rendered on the server
The HTML rendered on the server differs from the first client render, so React (Next.js/Remix) can’t attach to it.
Warning: useLayoutEffect does nothing on the server
A component using useLayoutEffect was server-rendered, where layout effects can’t run.
Warning: An update to Component inside a test was not wrapped in act(...)
A component updated state after the test’s synchronous assertions — async work wasn’t awaited.
error: cannot lock ref 'refs/remotes/origin/x': is at … but expected … / unable to update local ref
Git couldn’t update a reference — stale remote-tracking refs, case-only branch name clashes, or a leftover lock file.
warning: ignoring broken ref refs/remotes/origin/HEAD
A ref file is empty or points to a missing object — usually refs/remotes/origin/HEAD after the default branch changed.
Warning: componentWillMount has been renamed, and is not recommended for use.
A class component uses legacy lifecycle methods (componentWillMount/WillReceiveProps/WillUpdate).
Error: Objects are not valid as a React child
You tried to render a plain object (or a Promise/Date) directly in JSX; React can only render strings, numbers, elements and arra…
Error: A cross-origin error was thrown. React doesn't have access to the actual error object in development.
An error happened in a script loaded from another origin, so the browser hid the details from React.
Error: Invalid hook call. Hooks can only be called inside of the body of a function component.
A hook ran outside a function component/custom hook, conditionally — or two copies of React are loaded.
React Hook useEffect has a missing dependency (react-hooks/exhaustive-deps)
ESLint warns that the effect reads a value that isn’t in its dependency array, so it may use a stale value.
Error: Actions must be plain objects. Instead, the actual type was: 'function'.
A function (thunk) was dispatched but the thunk middleware isn’t configured.
java.util.ConcurrentModificationException
A collection was modified while being iterated (even in a single thread).
HTTP 412 Precondition Failed
A conditional request header (If-Match, If-Unmodified-Since) didn’t match the current state of the resource.
HTTP 428 Precondition Required
The server requires a conditional request (If-Match / If-Unmodified-Since) to prevent lost updates, and you didn’t send one.
HTTP 422 Unprocessable Content
The request is well-formed, but the data fails validation (e.g. invalid email, missing required field).
A non-serializable value was detected in the state / in an action, in the path
Redux Toolkit’s middleware found a Date, Promise, class instance or function in state or an action.
API Error: Resource already exists (ALREADY_EXISTS / resource_already_exists / 409)
The create request was refused because an object with the same unique identity — name, email, external ID — already exists.
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