OK so I've been doing quite a bit of digging and I'm now starting to lean towards El Grap's interpretation of the law. This website which appears to be run by referees suggests that an accidental trip is not a foul. Furthermore, my initial interpretation of 'acted without precaution' is probably wrong, in that it means he did not take care to avoid doing something he did, such as lifting his foot before stepping on the opponent's foot — if such avoidance was possible. In other words, if someone steps on another's foot without malice but still stood on it anyway he acted without precaution. It would seem in the instance of an accidental trip from behind there really is no reasonable way for him to have avoided tripping as he had no idea he was doing it in the first place (if that makes sense?).
Also I don't think you can say Alessi impeded Krishna as the definition is "Impeding the progress of an opponent means moving into the path of the opponent to obstruct, block, slow down or force a change of direction by an opponent when the ball is not within playing distance of either player." Alessi never moves into Krishna's path.
So I now think as by the letter of the law, O'Leary got it wrong. However I've seen this kind of decision so many times that it really appears that this law is interpreted differently by different referees, which is not surprising considering the debate we're having here!