When a visitor clicks Send, Contact Form 7 runs a tag parser. The parser checks every form tag for correct syntax and required attributes. If any tag fails the check, the submission stops and a red banner appears.
The parser rejects tags that lack opening or closing brackets, contain stray spaces, start with a number, or use disallowed characters such as colons or periods. It also flags tags that miss required options, duplicate HTML id values, or belong to a custom add‑on that is not registered with the current CF7 version.
The error message does not name a specific field. Instead it shows “(validation error) Invalid form tag.” The same text appears in the AJAX JSON response and in the WordPress debug log when debugging is enabled.