A user got thrown out of my form

I just had a user try to fill out a form, but when he filled in one of the answers, it threw him out. I received a notification with just that answer. Can you tell me why that happened? 

Is it possible that he accidentally pressed the enter key while on a textbox field? This normally initiates the submission of the form. Or he may have accidentally pressed the submit button by mistake.

I would suggest to make certain fields be required (especially those that you need to know), in order to prevent the submission of form with blank fields from happening. 

If you have questions, please let us know.

Yep - I bet that's it. I didn't have any answers set as required. Fixed now. Appreciate your quick response.