@test: Works as expected, after submitting the form I get the error, after applying patch the error no longer shows and form is sent successfully.

I followed the following test instruction:

1. Install a new captcha in joomla (other than recaptcha). I downloaded this captcha: http://extensions.joomla.org/extensions/extension/access-a-security/site-security/jgraphic-captcha-protection It has the name jgraphiccaptchaprotection
2. Enable multiple captcha plugins in Joomla plugin manager (recaptcha and jgraphiccaptchaprotection)
3. Open the file components\com_contact\models\forms\contact.xml

4. Go to the end of the file and find the captcha field. Add a new attribute in captcha form field as:

   plugin="jgraphiccaptchaprotection"

5. The field now looks like:

   <field
           name="captcha"
           type="captcha"
           plugin="jgraphiccaptchaprotection"
           label="COM_CONTACT_CAPTCHA_LABEL"
           description="COM_CONTACT_CAPTCHA_DESC"
           validate="captcha"
           namespace="contact"
   />

6. With sample data installed, go to this URL index.php?option=com_contact&view=contact&id=1&Itemid=229

7. When form page is loaded, the captcha "jgraphiccaptchaprotection" appears properly.
8. When the form is submitted, it throws the "Serialization of 'SimpleXMLElement' is not allowed" exception.
9. Apply the patch
10. Try sending the form again, this will now succeed.

@test successful. Thanks for the fix @radiant-tech and the test instructions @roland-d

I'm going to mark this RTC