Shooting a gun at a very easy target without warning is taking a potshot. And if you meanly and randomly criticize someone, that's another kind of potshot.
An overly easy criticism that's not well thought out is a common type of verbal potshot: "Your review was just a bunch of potshots at terrible movies!" This type of potshot comes from the gun-related version, first recorded around 1836. A potshot (or pot-shot) was a shot at an animal meant to "get it in the pot," or kill it strictly for food, rather than for the sport of hunting — the implication being that sport shooting required more skill.