Dollar Trading
I used to work at a small town bank with an ancient computer system. We had a small stash of Canadian currency that we would occasionally exchange for customers free of charge either way.
To do this, we looked up the current exchange rate and used our teller software to give us the value of $XX.XX CAD to USD or vice versa. I found out one day that this exchange rate NEVER updated and was stuck at 0.72 CAD to every 1 USD.
This was back when the USD was lower in value than the CAD. Being the enterprising individual that I was, I exchanged all of my cash (not much at the time, I was 16) for CAD with the excuse that I was going there over the weekend.
Went to a bank down the street and traded that money in for an easy 50% gain or something like that. I did this over and over for at least a year until they finally fixed the issue with the computer.
Story credit: Reddit / flinters17