There are always a few things that makes it easy to separate the locals from the tourists in a city. When it comes to New York, the behaviour on the public transport is one of those things. It's always a bit fun to watch people walking in the wrong direction, not finding the stop button on the bus or not understanding how to open the back door – wait for the green light and then touch the yellow strip on the door.
My first opportunity to make a mistake was because I didn't read the sign at the subway entrance properly. On many stops you have to choose the right entrance in order to get to the train going in the direction you want. Once you've passed with your ticket and realize your mistake you can't pass on the same ticket at the other entrance. Either you have to buy a new one or wait 18 minutes, the reason for this is of course to avoid people cheating and riding several people on the same subway pass.
Other possibilities for confusion are the express trains.In some ways you can read signs that says that the Q is an express while the N and R are not, but exactly what stops they are making is harder to know and sometimes it varies between weekdays and weekends. It's also possible that a train the is express in Manhattan becomes local in Brooklyn.