Why do I keep getting "Unconfirmed booking" emails?

Sometimes you will receive an email starting: "Attendee unable to complete booking". This article explains why...

This email is sent to you (the admin) when one of your attendees hasn't been able to find their booking confirmation email. There's a small link on the pre-confirmed booking page at the bottom of the list of possible reasons for them not having found the email. Clicking this link is what causes this email to be sent and is also preceded by an additional warning to only send the mail if they really can't find the email.

See the attached screenshots to see the steps your attendees have followed to send this mail to you.