“A extension has prevented this form from being submitted.”

[Gallaecio]

Found during translation. For consistency, it should probably be plugin instead of extension.

[ewinslow]

Is this shown to end users? I don't think we'd want to expose either of those terms to them.

[mrclay]

Displayed if the action_gatekeeper:permissions:check hook returns falsy. Ideally that hook's handler would set its own error message and forward away.

We could leave it "Sorry. Your form could not be submitted for an unknown reason." Not very satisfying :)