It's where you accidentally wave to people who weren't actually waiving at you.
It's where you don't know what to do with your eyes when passing a stranger on the sidewalk.
Crossing the street at the incorrect time can be fatal.
You have to accurately gauge your level of enthusiasm for greeting people you know but might actually care nothing about. In a worse case scenario, you run into a little-remembered acquaintance and find out you're going inside the same supermarket where you will be wandering around for some time. Then you have to decide if you're going to pretend to be genuinely interested in a cottage cheese label or awkwardly acknowledge that person again.
This is why I hate going outside....