That's not at all a mistake. Any sort of overshoot is extremely unlikely to occur with these situations. The public plan was to put it into an orbit around the

That's not at all a mistake. Any sort of overshoot is extremely unlikely to occur with these situations. The public plan was to put it into an orbit around the sun with the perihelion near Earth's orbit and its aphelion near Mars's. Using the additional remaining fuel, they decided to burn until the aphelion reached well past mars towards the asteroid belt.