Revealing mistake: In the big wheel, the boy falls out and grabs Axel's leg. When it zooms out, it is a man climbing his leg.
Continuity mistake: After Inspector Todd is shot, Axel chases after De Wald and fires at the truck. When the guy at the back of the truck is shooting at Axel, we see multiple bullet holes near the truck's back right window, but in his close-ups the bullet holes are gone. The window frame's scratched paint differs as well in the close-ups. We know it's the same window because we also see him in the shot of the truck's interior.
Answer: Just before Uncle Dave is shot earlier in the film Axel figures out that the note he was given was written on treasury mint sample paper - I believe it's at this point that Axel know Fulbright was in on it since the "matter of life and death" note was written some time before the treasury mint paper truck heist.