Some valid form submissions are blocked and EE errantly thinks they are robots

Question

I have several contact forms on our site, but only experience issues with the form for job applications (http://www.autouplinktech.com/careers). Most submissions work with no problems and I cannot reproduce the issue, but we have many seemingly random report of users not being able to submit the form because EE mistakenly thinks they are a robot. We have no captcha enabled on the form whatsoever.

Anyone experience this sort of issue or might know what to do?

We are using Freeform Pro for the form.

Answer 1

So we've had some good feedback from a user to let us know what the problem is most likely. The issue was with our job application form on the Careers page. Users were attempting to submit their info from a 3rd party form like Flexjobs.com. When they attached their resume, the form attempted to submit their info with the resume as an attachment. Freeform just adds the resume to an Uploads folder which of course can only be accessed by our local system. EE or Freeform didn't know what to do with an attachment from a 3rd party form submit, so it treated like like a spam attack and blocked it.