I did this to get free laundry in uni. It read the balance from your card and then wrote the new balance. I discovered you could swap cards at the right point and write the new balance to any card, thus getting free money.

I got the idea from a great book "Security Engineering" which IIRC mentioned some people in Africa doing the same trick with electricity cards or something. Might have misremembered.

That book had a foreword saying something to the effect of "Should this book be written? Some say people will use it for nefarious purposes, however..."

Sorry authors, I used it for nefarious purposes. :D