Besides the fact that NYC is (in parts) a gritty, decaying, dirty, and disorderly place where you are very likely to meet with formidable foot hazards such as broken glass, chunks of brick and concrete, nails, tin cans and other refuse on streets and sidewalks, if you take the subway late at night or during off-peak hours when relatively few people are around you might have the following experience:

One hot night, I was wearing flip flops while waiting for the train when I felt something lightly scratch my foot. I looked down and there was this huge rat who had come up onto the platform and walked across my unprotected, flip-flopped feet. I have since heard that there are seven rats for every New Yorker, so this kind of thing must happen a lot.