When an email is sent to a known prospect (prospect A), and the prospect then forwards that email to someone else (prospect B) and that person clicks on a tracked link, it can result in a successful form or landing page completion in prospect B's activities even though they may be missing some of the data from required fields in the form.
The links within emails sent through Pardot or using our email plug-ins are tracked to specific individuals. In this case, prospect B clicked on an email meant for prospect A. Since the system thinks that prospect A is viewing the form, it only displays fields for which it does not already have data (we always ask for email address). So in the case of prospect B, this is all that he/she would have seen.
When prospect B corrected the pre-filled email address (i.e. prospect A's email) and submitted the form, we created a new prospect record rather than overwriting prospect A's record to prevent merging issues where data may have been overwritten.
One way to help prevent the scenario above from occurring is to include the "Not You?" link on your forms. This is available in the Form Wizard in the Look and Feel step on the Advanced tab. If a cookied user (i.e. someone already being tracked or clicking a tracked link) encounters one of your forms, a "Not you?" link will appear under the email field. If they click it, it resets the form and displays all the fields.
Additionally, if you are using conditional fields in your form, they will only display after the fields they are dependent upon are completed.
Another option is to set your form to kiosk/data entry mode. If this option is enabled and the browser already has an existing visitor/prospect cookie, this information will be ignored and progressive profiling will not be activated. Forms and form handlers with this option enabled will record the prospect submission and run completion actions, send auto-responders, etc just like normal. However upon viewing and submission, the browser will not be cookied or tracked. For more information on enabling kiosk/data entry mode, please see Kiosk/Data Entry Mode for Forms, Form Handlers and Landing Pages.