I remember once I was buying Burt's Bees Lip Balm (I'm addicted!) at Bath and Body Works and I was rushing and not paying attention because I had a plane to catch later that evening for a long trip. I grabbed one for $2.50 and as the cashier was ringing me up, she said, "Do you want another one? They are two for $5.00 so you might as well!" Not thinking at all, I grabbed it. As I was walking out, I realized that this was not a deal, but just simple math.
Lesson Learned: Don't go shopping when I am incapable of 1st grade math.