Error map
BOM output Unexpected characters at the top of the page / headers already sent by BOM
PHP · 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)
URIError: URI malformed
decodeURIComponent/decodeURI received an invalid percent-encoded sequence.
CORS Error: No 'Access-Control-Allow-Origin' header is present
The browser blocked a cross-origin request because the server’s response didn’t include the CORS headers allowing your site’s ori…
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.
HTTP 411 Length Required
The server refuses a request with a body but no Content-Length header.
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…
Warning: Cannot modify header information - headers already sent
You called `header()`, `setcookie()` or `session_start()` after output had already been sent to the browser.
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.
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…
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 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 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…
Two steps away (35)
HTTP 404 Not Found
The server can’t find anything at the requested URL.
AbortError: The user aborted a request. / signal is aborted without reason
A fetch was cancelled through an AbortController (or the browser cancelled it).
TypeError: Failed to execute 'json' on 'Response': body stream already read
A fetch Response body was consumed twice (e.g. calling .text() then .json()).
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.
HTTP 401 Unauthorized
The request has no valid authentication credentials — you need to log in or send a valid token.
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…
AWS: ExpiredToken — The security token included in the request is expired
Temporary AWS credentials (STS/SSO/assumed role) have expired.
Warning: session_start(): open(/var/lib/php/sessions/sess_x, O_RDWR) failed: Permission denied
PHP can’t write session files to the configured session.save_path.
InvalidOperationException: Session has not been configured for this application or request
Code used HttpContext.Session, but session services/middleware aren’t registered — or UseSession runs after the endpoint that nee…
HTTP 400 Bad Request
The server could not understand the request because its syntax, headers or body are malformed or invalid.
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.
API Error: Missing required parameter (parameter_missing / "x" is required)
The API rejected the request because a field it requires — in the query string, path, headers or JSON body — wasn’t sent or was e…
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…
Upload error: File type not allowed / unsupported file type
The upload was rejected because the file’s type — extension, MIME type or actual content — isn’t on the allowed list.
HTTP 417 Expectation Failed
The server or a proxy can’t meet the Expect header in the request — almost always Expect: 100-continue.
Cloudflare Error 1035 / 1036: Invalid request rewrite (invalid URI path / query string)
A Transform Rule (URL rewrite) produced an invalid path or query string.
Cloudflare Error 1037: Invalid rewrite rule (failed to evaluate expression)
A rewrite rule’s dynamic expression failed at runtime — for example a function received a value of the wrong type or a missing fi…
HTTP 500 Internal Server Error
The server hit an unexpected condition — usually an unhandled exception or misconfiguration in the application.
PHP Fatal error: Uncaught Error: Call to undefined function
PHP can’t find the function you called — usually a missing extension, missing include, or namespace issue.
Warning: Undefined array key / Undefined index
Your code reads an array key that doesn’t exist — common with `$_GET`, `$_POST` and decoded JSON.
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.
Script error. (at line 0) — cross-origin error details hidden
An error happened in a script from another origin, and the browser hid the details.
HTTP 504 Gateway Timeout
A proxy or gateway waited too long for the upstream server to respond and gave up.
Cloudflare Error 524: A Timeout Occurred
Cloudflare connected to the origin, but the origin didn’t send an HTTP response within 100 seconds.
HTTP 304 Not Modified
The cached copy is still valid, so the server sends no body and the client reuses its cache.
AttributeError: 'X' object has no attribute 'y'
You accessed an attribute or method that doesn’t exist on that object type.
ERROR: invalid byte sequence for encoding "UTF8": 0x00
Text contained bytes invalid in UTF-8 — often a NUL byte (0x00) or Latin-1/Windows-1252 data.
UnicodeDecodeError: 'utf-8' codec can't decode byte 0x92 in position 10: invalid start byte
Python read bytes that aren’t valid in the encoding it assumed (usually UTF-8).
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.
java.lang.InterruptedException (sleep interrupted)
A blocked thread (sleep, wait, join, queue take) was interrupted by another thread.
java.util.concurrent.TimeoutException
A Future.get(timeout) or async operation didn’t complete in the allowed time.
HTTP 412 Precondition Failed
A conditional request header (If-Match, If-Unmodified-Since) didn’t match the current state of the resource.
gzip: stdin: not in gzip format / tar: Error is not recoverable: exiting now
The file isn’t actually a gzip archive — often an HTML error page saved by curl/wget, or an archive with a different compression.
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