When I'm logged in as an Administrator and I try to "Add New User", it says "ERROR: Please fill in the reCAPTCHA form."
But there shouldn't be a reCAPTCHA form. This is not a self-registration form. This is the admin's registration form.
The workaround is for me to disable the setting "Enable for registration form" temporarily and then re-enable it.
I'm running WP 3.0.5 multisite.