It happens more often than you think. People need to sign something before a notary as witness but they sign the documents before they come in. I then have to request that they sign the document again so I can witness the signing. It is not a good look to have two signatures on a page. Otherwise we have to take steps to get a blank version of the form for them to resign. Unless they happened to have a spare handy, this involves accessing the internet from my office and reprinting or accessing via email.
Sometimes a customer has to phone back to their office to have someone email me the document. If the form has preliminary details to complete before signing (name, address and so forth) and customers then have to fill them in whilst at my office. All this takes up professional time and does not assist me to stick to price guidelines for documents. In summary if your document needs a witness then you must allow your witness to witness you signing.