Stupidity: When they're tied up in the lab it's not that hard to escape. They have this loose straps with clips. (01:35:40)
Stupidity: While in Romania, Ali was a marine sergeant who was able to single-handedly take down multiple villains approaching from all sides by accurately firing one shot per villain and using vehicles for protection. At the Carrington Hotel in New York, Ali and villain Borz run toward each other while repeatedly firing their guns, to no avail. They meet in the middle, Borz knocks Ali onto the floor, then waits until Ali stands up before resuming the fight. A little later, Ali is on the floor again and tosses a hand grenade toward Borz's feet, and Borz just stands there and gets blown up. (01:28:18 - 01:28:51)
Stupidity: Sam slid under the truck to narrowly escape the lioness. Even though there was more than enough ground clearance, the lioness did not go under the truck to attack Sam, as though the truck's frame were a barrier. The lioness could have easily crawled under the truck (unless a CGI lioness is not as spry or nimble as a "real" lioness). Sam being under the truck would not constitute adequate protection from an attacking lioness. (01:10:30 - 01:35:20)
Stupidity: A zombie is climbing up the apartment wall and is almost at Aidan's balcony railing. Instead of trying to push the Zombie off while he had the advantage (and there was a surfboard on the balcony that could have been used), Aidan ran inside and locked the door to the balcony. This furthered the plot and added excitement by having zombies coming at Aidan from multiple directions. (00:43:09)
Stupidity: Early in the movie, AMMO and its crew show remarkable prudence in their approach to the drug deal, and a point is made about them following procedure, planning accurately using technology and trying to minimize collateral damage. The sting at the club goes against all of that. At the end of the day, it just consisted of getting close to the target and pulling guns on him in the middle of a crowd of civilians, the exits of the place (the front door even) completely unguarded.