The company I work for is located on the 2nd floor of a building. There is a pretty nice restaurant on the first floor, as well as a few high-dollar shops. I'm a semi-regular at the restaurant for lunches, at least for to-go orders.
Around a month after I started at the company (last summer), I was ordering a slice of cheesecake from the restaurant as an afternoon treat maybe 2x a week. One day I went down, cash in hand and the manager refused to take it. He told me that I was on the "buy 100 slices, get one free" plan.
A few weeks ago, I was running late for work and left my lunch sitting on the counter. No big deal, right? I'd just order lunch from downstairs. I go down to get my lunch, and not only is my debit card not in my wallet (I found it on the desk at home that night), but my gift certificate from the restaurant wasn't in my purse either. I had JUST enough cash to cover lunch. While paying, I laughed and said that I must have left my brain at home. I told the manager that I'd not only forgot my lunch and debit card at home, but that my gift certificate for the restaurant was probably on my desk as well. He handed me my cash back and told me to bring it in the next day, not to worry about it.
I've surprised them too, a few times. I bought lunch one day, using my debit card. When the transaction hadn't cleared after a week (it usually clears after 2 days - I checked my transaction history), I went down to ask the manager about it. He took the transaction info and said he'd investigate. The next morning he came up and told me that after the host ran my card, it somehow ended up being processed as cash, but that they weren't going to process my card again as their books had not only balanced for that day, but came out showing that they'd earned more than they'd sold. He thanked me for my honesty though. I told him that if my checkbook wasn't balancing, I figured theirs wasn't either and I wanted to pay for the food I'd enjoyed.
He wouldn't let me though.