Error map
CORS CORS Error: No 'Access-Control-Allow-Origin' header is present
API · layer 4, Application & API
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)
HTTP 403 Forbidden
The server understood the request but refuses to authorize it — you (or the server process) are not allowed to access this resour…
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…
HTTP 401 Unauthorized
The request has no valid authentication credentials — you need to log in or send a valid token.
GraphQL: Cannot query field "x" on type "Y"
The query asks for a field that doesn’t exist in the server’s schema (or isn’t available on that type).
Warning: session_start(): Session cannot be started after headers have already been sent
session_start() ran after output was sent, so PHP can’t set the session cookie.
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.
Google Maps: This API project is not authorized to use this API / RefererNotAllowedMapError / ApiNotActivatedMapError
The Maps API key isn’t allowed for this site/API — the API isn’t enabled, the referrer restriction doesn’t include your domain, o…
Unexpected characters at the top of the page / headers already sent by BOM
A UTF-8 byte-order mark (BOM) at the start of a PHP file is sent as output, breaking headers, JSON and images.
HTTP 100 Continue
An interim response telling the client to go ahead and send the request body — normal when the client sent Expect: 100-continue.
Cloudflare Error 1040 / 1041: Invalid request header modification
A Transform Rule tried to set or remove a request header that Cloudflare doesn’t allow to be modified, or produced an invalid val…
SignalR: Error: Failed to complete negotiation with the server / Failed to start the connection
The SignalR client couldn’t negotiate with the hub — wrong URL, CORS, authentication, or a proxy that blocks WebSockets/negotiate…
Swagger UI: Failed to load API definition. Fetch error: Internal Server Error /swagger/v1/swagger.json
Swagger UI couldn’t fetch or parse the OpenAPI document — the generator threw an error, the URL is wrong, or CORS/auth blocks it.
Two steps away (36)
Git: Permission denied (publickey). fatal: Could not read from remote repository.
SSH authentication to GitHub/GitLab/Bitbucket failed — no key, wrong key, or the key isn’t added to your account.
AWS S3: AccessDenied (403) when calling GetObject / PutObject
An IAM policy, bucket policy, ACL/ownership setting, Block Public Access or KMS key policy denied the S3 request.
Linux: Permission denied
Your user (or the service user) lacks read/write/execute permission on a file, directory or one of its parents.
Error: connect ECONNREFUSED 127.0.0.1:5432
Nothing is listening at that host:port — the target service is down or you’re connecting to the wrong address.
SyntaxError: Unexpected token < in JSON at position 0
`JSON.parse()` / `response.json()` received something that isn’t JSON — very often an HTML error page.
SSL: CERTIFICATE_VERIFY_FAILED / unable to get local issuer certificate
The client couldn’t verify the server’s TLS certificate chain, so it refused the HTTPS connection.
Kubernetes: ImagePullBackOff / ErrImagePull
The node can’t pull the container image — wrong name/tag, missing registry credentials, or registry unreachable.
Azure AD (Entra ID): AADSTS700016 Application not found in the directory
The client ID in your sign-in request doesn’t exist in the tenant you’re authenticating against.
HTTP 400 Bad Request
The server could not understand the request because its syntax, headers or body are malformed or invalid.
HTTP 200 OK (but something is still wrong)
The request succeeded at the HTTP level — yet the page is blank, the data is wrong, or an error is hidden inside the response bod…
MySQL ERROR 1005 (HY000): Can't create table (errno: 150 "Foreign key constraint is incorrectly formed")
A foreign key definition is invalid — the referenced column’s type, index or charset doesn’t match.
MySQL ERROR 1007 (HY000): Can't create database 'x'; database exists
CREATE DATABASE ran for a database that already exists.
AWS: ExpiredToken — The security token included in the request is expired
Temporary AWS credentials (STS/SSO/assumed role) have expired.
Warning: Cannot modify header information - headers already sent
You called `header()`, `setcookie()` or `session_start()` after output had already been sent to the browser.
HTTP 411 Length Required
The server refuses a request with a body but no Content-Length header.
Mixed Content: The page was loaded over HTTPS, but requested an insecure resource
An HTTPS page tried to load a script, API or image over plain HTTP, and the browser blocked (or warned about) it.
SecurityError: Blocked a frame with origin "x" from accessing a cross-origin frame.
Script tried to read or modify an iframe (or window) from another origin.
Refused to load the script because it violates the following Content Security Policy directive
The page’s Content-Security-Policy doesn’t allow that script, style, image or connection source.
API Error: Invalid API Key / Incorrect API key provided
The API didn’t recognise the key sent with the request — it’s wrong, revoked, from another environment, or sent in the wrong plac…
API Error: Too many concurrent requests (concurrency limit reached)
You have more requests in flight at once than the API allows for your key — even if your requests-per-minute rate is fine.
API Error: Feature not enabled for this account (feature_not_enabled / not available on your plan)
The endpoint or option exists, but your account, plan, region or project hasn’t been granted access to it.
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.
URIError: URI malformed
decodeURIComponent/decodeURI received an invalid percent-encoded sequence.
UnicodeEncodeError: 'charmap' codec can't encode character '\u2192'
Python tried to write text in an encoding that can’t represent some characters — common in Windows consoles and files opened with…
HTTP 413 Content Too Large (Request Entity Too Large)
The request body is bigger than the server is willing to accept — usually a file upload over the configured limit.
HTTP 505 HTTP Version Not Supported
The server doesn’t support the HTTP protocol version used in the request.
WordPress: The link you followed has expired
Usually an upload (theme, plugin, media) larger than PHP’s upload/post limits — WordPress receives an empty request and shows thi…
HTTP 103 Early Hints
An interim response that sends Link headers early so the browser can preload CSS, fonts or scripts while the server is still buil…
HTTP 416 Range Not Satisfiable
The client asked for a byte range (Range: bytes=…) that lies outside the file — usually a resumed download of a file that changed…
HTTP 417 Expectation Failed
The server or a proxy can’t meet the Expect header in the request — almost always Expect: 100-continue.
WebSocket connection to 'wss://x' failed
The browser couldn’t open (or keep) a WebSocket connection.
ObjectDisposedException: Cannot access a disposed context instance
Code used a DbContext after its scope ended — usually async void, fire-and-forget tasks, or capturing a scoped context in a singl…
ASP.NET Core: The CORS protocol does not allow specifying a wildcard (any) origin and credentials at the same time
The CORS policy uses AllowAnyOrigin() together with AllowCredentials(), which browsers forbid.
Error: socket hang up / read ECONNRESET
The other side closed the TCP connection abruptly while your client was still using it.
Error: getaddrinfo ENOTFOUND host / Name or service not known
DNS lookup for the hostname failed — the name is wrong, doesn’t exist, or this machine can’t resolve it.
API Error: Job / operation not found or expired (async task ID unknown)
The status URL or job ID for an asynchronous operation (export, import, render, batch) doesn’t exist any more, or never existed.
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