In the first episode, Gi-hun tries accessing his bank account and finds that his mother has changed the PIN after he tries inputting his usual password and it doesn't work. His usual password is his birthday, which we see is 0426 or April 26th. However, in the final scene of the last episode of the first season, he identifies himself on the phone with his name and birthday, which he gives as October 31st.
In Season 1, Episode 3, at 18:35, (Player 212) is shown going to the bathroom. Although she is supposed to be hiding the cigarettes and lighter deep down inside a tampon, the pink color of the box can be seen before she even gets it out.