During one round of marbles between Gi-Hun and the old man, the old man wins. Gi-Hun has 3 marbles in his hand, the old man 4. But when Gi-Hun goes to dump his marbles into the old man's hands, now he has 4 and the old man has 3.
Three contestants disappear without explanation in this episode. 38 enter the arena and half of them die there leaving 19. Player 212 gets a free pass as well making a total of 20 survivors but when they all return to the dormitory the board shows 17 remaining. However, that just means that there were three pairs of people in which neither player was able to win all marbles of the other within the time limit, so all six were eliminated.
When #456 is playing marbles against #001. The old man bet 5 balls on odds and then it shows he won 4 balls from #456 then the scene cut to #456 handling 5 balls instead of 4.
The stamp seen on the non disclosure agreement found by Hwang Jun-ho reads "TOP SCRERT".
This has since been fixed by Netflix.