Stupidity: Why does Smith even take along the traitors to begin with, instead of simply killing them outright? He got the information from them he wanted. He didn't need them for anything else. Was it to make them stand trial instead of summarily executing them? Even after he's killed dozens of other people for less? But on the other hand, you wouldn't have had the excuse for the gondola fight that was a film highlight.
Stupidity: Why didn't the Major turn the gondola around rather than jump onboard and confront the spies?