Goofs

Jefferson has sparrow tattoos on his shoulders. In scene after his guard tries to kill him, these tattoos disappear and re-appear shortly after.

After Vince is shot on the train he has blood on both hands when he is in front of the glass. In the next shot his left hand is clean.

An exit door in the train station is locked preventing exit. Safety building codes require exit doors to be open from the inside even if they are locked when trying to enter.

The cylinder in a properly functioning revolver will not spin freely once closed. While normally correct, the double-action revolvers used for the competition were owned and distributed by the holder(s) of the competition, and were likely modified for use in the competition. The modification is a simple one, and considering the stakes and as a necessary element of the competition, it is very likely the modification was made. It is obvious during the competition, that the cylinders are, in fact, able to spin when closed, despite common functional design.

The key taped to the letter at about 0:12 is an uncut blank.

The idea of a player being "in suspense" during the duel stage of the tournament is completely nullified by the revolver's cylinder being open/visible at the front. The opposing player would be able to easily tell if there is in fact a bullet lined up in the other man's gun.