At the part where Nikki is at the vet to drop her dog off, Ross is outside on the pay-phone leaving a message on Amy's message machine. Now if you watch inside the vet from the window as they are showing Ross on the phone, you will not see anyone in the vet. Even tho there is a guy sitting on the red bench and Nikki is standing right at the front of the reception desk. And they switch from watching Ross and Nikki back and forth a few times, and that basically confirms that the guy and Nikki are still in the same spots, but yet aren't when they are showing Ross outside.