Bronson makes his darts before knowing what he will use as a tube to know the size of darts. He disassembles a lamp to use as a blow gun that just happens to be the exact size of the darts that he makes.
During the cabin scene, when Charlie nails Cerutti in the back with the hatchet, Cerutti plunges off the roof dead...except when his character lands faced down in the snow, you can see him exhale by the movement of snow near his mouth.
When Charlie (Charles Bronson) throws the ax at Cerutti (John Hallam) at 54:34, the ax goes past Cerutti's left shoulder before striking him in the back.
When the viewer sees through Huntz's rifle scope at Jackie, what is seen is from the ground, not from the scaffolding, what 5 stories up? 1:26:16.
The dead girl's eyelashes move in the scene at the morgue at 1:55.