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: Major Smith takes an incredibly unnecessary risk in the tavern by telling an officer that he is Himmler's brother. Heinrich Himmler was chief of the SS and news of his brother's presence in the tavern would have spread like wildfire, especially amongst the SS and Gestapo officers who would have wanted to come and say hello, potentially exposing the group. The group eventually gave themselves up in the tavern but revealing himself as Himmler's brother could have got Smith caught before he had time to get out to the woodshed to meet Mary and Heidi. (00:36:13 - 00:36:50)
Stupidity: Why didn't the Major turn the gondola around rather than jump onboard and confront the spies?