Stupidity: Why did the bad guys even go for Xander as he was snowboarding down the mountainside? First of all, it was impossible for them to even get to him as the mountainside had several cliffs. Secondly, why did so many go? Wouldn't it be clever to have as few as possible leave in case his snowboarding was simply to draw attention away from an ambush? They knew he was an agent by now and should know he could have gotten reinforcements. Thirdly, they could just have simply waited for him to get to them.
Stupidity: At the start, when the bank robbers pile into the car, Frank flips a switch and the licence plate changes...out in full view of the public. Given how well-prepared Frank is demonstrated to be, that's absolutely something he would have done in private, before arriving at the bank.
Suggested correction: The plate is changed after the robbery for a reason. By allowing plate A to be seen during the robbery the information will be relayed to the police that a car with registration plate A is involved. Swapping to plate B after the robbery ensures that police will see plate B and know that the car is not involved.
But the point is he drives up to the bank with plate A on display. The robbers pile in and he switches to plate B. So any witnesses have seen both plates (plus the fact that the car switched plates, doubly suspicious). Whereas if he'd switched to plate B while out of view in the car park, he'd still have a "clean" plate to use.
Stupidity: When the policeman comes for the girls, the people run along a straight path, completely straight, without bothering to go over the fence right next to them or run in a different direction.
Stupidity: When the police turn up the first time the mother lies to them to get them to go away but the burglars can't hear the conversation. She could have told them exactly what was happening and that she had to make it look like she'd got rid of them. She could have then smashed all the cameras and just let them in.
Suggested correction: This was a character decision, and a completely understandable one. Her daughter was being held hostage in the panic room. She decided to cooperate with their demands for the safety of her daughter. Your idea is a solid one, but she simply did not think of it in her state of mind. She opted to get rid of the police, and her decision is not a movie mistake.
Stupidity: In the film climax, Cassie tells Justin to put his gun on the floor and walk over to her. This he does but it gives Richard the chance to grab the gun, so we can have the usual daft Hollywood ending. Why didn't she simply tell him to kick the gun over to her first to void this?