Is this a PDF-based form? Are the responses submitted via FormsCentral, or is the form simply filled out and then sent to you?
Thank you very much Brian,
The form is filled out then sent back to me.
My guess is that people are using a non-Adobe PDF editor to complete the form: Preview, NitroPDF, iAnnotate, etc. You may need to be explicit about only using Acrobat Pro or Adobe Reader to complete the form. I see the same thing from time to time.
I hope that helps,