Revealing mistake: This mistake occurs in the beginning of the film where the bandits rob the train. When Jackie is standing on the coal (or was it wood?) car you can see the train is passing a modern sign (you can see it on the right).
Revealing mistake: At the end, when Jackie is fighting the bad guy, he jumps up into a windowsill and then jumps down when the bad guy tries to hit him with a spear. If you look at the spear when it hits the wall, you can see that the spearhead is made of rubber because it bends when it touches the wall.
Revealing mistake: When Roy and Chon are escaping from jail, Chon uses all his strength to bend one of the iron bars aside so that Roy can squeeze through. But, as Roy squeezes through, the iron bar behind him (which should be rigid) also visibly flexes, revealing that the bars are made of rubber or plastic tubing.