783 Unexpected Token
Meaning
A non-standard status code used by Shopify to indicate that the request contained a JSON syntax error. Essentially the server couldn't parse the JSON in the request.
When to Use
When a Shopify API endpoint receives a request with malformed JSON (invalid syntax), it may return 783 Unexpected Token to specifically denote that the JSON could not be parsed (unexpected token encountered in the JSON text).
Example
A client sends a POST to the Shopify API with a malformed JSON body (e.g., missing a comma or curly brace); Shopify responds with 783 Unexpected Token, meaning it found an unexpected token in the JSON and couldn’t parse the request body.
When Not to Use
Do not use in standard APIs. The proper response for a JSON parse error in a request would typically be 400 Bad Request with an error message. 783 is a very specialized code not recognized outside of Shopify’s context.
Source
Unofficial (Shopify)