This happens to me a lot, too (even when I'm not folding tables of shirts at the mall, which I have a habit of doing). One time, my husband and I were looking for something in the kitchen department at Walmart, talking to each other all the while. An older couple was also in that department, and the wife just started asking for help. Not having any idea she was talking to me, I didn't respond. Then, she looked at me a little more closely (not wearing anything close to the employee uniform) and asked if I worked there. She wasn't rude when I said no, which was nice. I had to laugh, though, because this isn't the first time I've been mistaken for an employee someplace where I've been obviously shopping.
"Redheads have at least a 95% chance of being gorgeous. They're also concentrated evil." - Irv
"This is all strange, uncharted territory and your hamster only has three legs." - Gravekeeper