Happens relatively often on the 757 and 767 where the cabin divider is in the mid cabin. I presume because the galley at D2 on both aircraft provides a more permanent physical divider between the two cabins and maybe people don't realize that the cabin extends into the second cabin.

On such flights we regularly count passengers as particularly coming back from places like DME and IST, we are catered to the exact number of passengers and it would cause problems if we were one or two over in passenger numbers.