As this article from 2012 makes clear, the positive return on investment resulted from unusual rules in one state lottery game that has now been discontinued. The typical state lottery in most parts of the United States has parimutuel pay-outs for winning tickets, meaning that if multiple purchasers have winning numbers, they split the prize in a way that ensures that the lottery doesn't pay out more money than was bet for that drawing. An example from Britain was a national lottery that had something like seventy-six different winners, who all bet on the same "lucky" number that happened to be drawn in that drawing. They split the prize equally, so that each bettor's individual winnings from a large prize were not particularly large. The number may have been lucky in the sense that it matched the drawn number on that one occasion, but the number wasn't INDIVIDUALLY lucky for each person who bet it.<p>Over and over and over, some people have winning tickets, but most people have losing tickets. When a lottery is structured in the typical parimutuel way, as most lotteries are, even if you buy all the tickets available for sale in the next drawing, which takes a big investment, you can't be sure of winning a full prize individually, because other bettors may have a collision with your choice of a winning ticket number. tl;dr: A bug in one state lottery game was discovered by MIT students, who invested in exploiting the bug until the game was closed.