reCAPTCHA asking to copy paste code but failing

A very small amount of my users get a captcha that asks them to copy and paste a code, but it always fails for them - while most of the users get the normal one (checkbox) which goes through correctly. Googling only returned three instances of people getting that captcha none of which had any valuable information Any ideas as to why they're getting that captcha and most importantly why does it fail? -- 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.