Unrecognizable Captcha coming

Hi, In my site reCaptcha implemented and working well. but today I found that continuous it displayed captcha which is not recognized by me and our other members, what to do for that? example captcha is  Thanx... :) -- You received this message because you are subscribed to the Google Groups "reCAPTCHA" group. To unsubscribe from this group and stop receiving emails from it, send an email to recaptcha+unsubscribe@googlegroups.com. To post to this group, send email to recaptcha@googlegroups.com. Visit this group at http://bit.ly/1dkFnYd. For more options, visit http://bit.ly/P65DvS.