Artificial Stupidity/Video Games/Puzzle Game

This trope was deliberately invoked for the PC puzzle game Sheep, where you play a herder driving a flock of sheep around various obstacles, and you had to compensate for the flock's tendency to go in the wrong directions and frequent suicidal stupidity. The game even lists this artificial stupidity as a feature.
The LEGO games are especially good (bad?) at this. Although it has improved in recent games, more than often your partner will be busy walking into a wall on the other side of the level, while you need him to come and use his ability.
Although it's understandable given the insane number of things you can work with, the AI in Scribblenauts will stick to its guns regardless of whether it makes sense or not. This leads to amusing situations like a group of witches accidentally transforming one another into frogs while fighting an enemy, and then the survivors eating their frog-ified sisters.
Stones Masters belongs here. Once the player's opponents get the "Dispel" spell (which removes attack and defense bonuses), the opponents will use it regardless of whether or not it will have any effect.
In Nuts & Milk, Nuts has a tendency to suicidally plunge into the water at the bottom of the screen, but makes up for it by Respawning Enemies quickly.