In the first episode, Gi-hun types his PIN code, 0426, and says it's his birthday, i.e. 26/04. But here, at the 3:50 mark, as seen on his computer file, his birthday is on 10/31. This is confirmed in episode 5 in his file in the binders.
Player #331 is first seen as a woman with glasses in the arena at the end of the first episode, then later as a man with glasses at the 13:20 mark.
Similarly, player #61 is first seen as an woman about to be shot during the honeycomb game, then as a man in the floor tiles when the corresponding tile goes dark.
When the policeman is hiding behind vans on the ferry, two of the vans have the same number plates.
The amount of honeycomb removed changes when Gi-hun is being watched by the guard.
During the candy-breaking scene, the initial shot of the playground area shows an umbrella on the wall with no grooves at the bottom. The next shot has the umbrella with grooves.
There is no way the needle would get in the guard's eye. The needle barely penetrated through the mask and the mask is too far away from his eyes, as can be seen by lighting up Jun-ho's face inside his mask in "Stick to the Team".
With workers above and on the same level as the vans, it is completely implausible that none saw the violently shaking van, during the scuffle with the detective.