What are common mistakes those new to living in NYC make?

My answer is : not realizing the importance of a quiet bedroom / apartment (or not realizing how an apartment facing a noisy street can ruin your life)

edit for those asking: I once lived on 6th Ave in the 20s for a few months and the frequent fire trucks and ambulances running up the street were beyond the power of any noise blocking tech and that ruined my sleep and my life till I left. Some people may be less sensitive to noise — I thought I was — until then