I would say only two main things can go wrong. Firstly, human error when you are matched betting. On this, I would advise, keep track of your bets on a spreadsheet. Double-check everything! Mistakes with matched betting can luckily go in your favour in a big way or something go the other way, so be careful. The other thing that can go wrong is betting websites working out that you are matched betting. If they do, they will restrict your account so you can no longer get any offers or bonuses. This is called gubbing. Get too many of these and your options for making a profit will reduce.