When you add an external link with VE to any page, you are asked to enter a CAPTCHA code when you save. You enter the code, hit "save page" and the "save your changes" box is reloaded with a new CAPTCHA and no error message. Edit is not saved.
Reported on ja.wp, I've also tried it on other wikis where I'm not autopatrolled atm, with different external links and with a proofreading of the CAPTCHA by someone else. Everything goes fine when I use wikitext editor.