Easy Mistake to Make?

This morning I stood patiently as the old gent in front of me was served at the Coop in (The) Forth.

"You need anything else, Bobbie?" asked the cashier politely.

"Naw, hen ............ I'll just get ma stuff and let ye serve the lassie."

The cashier looked at me, I looked at her, we both looked behind me....... nobody else ......... we both shrugged.

"Aw sorry son, thought ye were a lassie!" said the auld gent, chortling as he went on his way.

Boy Named Sue!