I did that yesterday at work! On purpose, though.
I was in the break room at work and I saw that there was a pack of M&Ms hanging by a thread. I tried shaking the machine to see if I could get it free but no luck.
So, I put my money in and got 2 for the price of 1. At $1.25, it's still overpriced but I feel like I stuck it to the man!