Just want the admin to know that this "security feature" is a real pain in the ass. It sometimes takes up to 5 minutes to finally pass. 
Well, that's how things work. Just keep trying until you successfully passed it. Let me tell you some tricks to prevent errors in Captcha. 1. Just make sure that you have a strong Internet connection, sometimes Captcha's keep showing errors because your Internet connection is way too slow. I've experienced it a lot when I'm having a turtle net.
2. If you don't want to face this Captcha every time. Just save your Bitcointalk.org's password in your browser. So that it won't be necessary to log-in everytime. But saving your password is obviously not safe if you're browsing Bitcointalk in computer shops. Your account might get compromised if left logged-in.
3. Just use a normal browser like (I highly recommend) Google chrome when browsing Bitcointalk. Using Tor sometimes gives an enormous number of errors when solving Captchas.
4. And the last but not the least. Make sure that you're a real human
Aliens and bots can't read Captchas, I'm afraid that you might be one of them.