Zombies can hit through the floor of a yellowish house that is located on the corner of roads in cities (like the apartment buildings, its at the end of the block usually). This is the house-ish hotel-ish thing with the curtain behind a counter in the front, and rooms upstairs.

Basically, if zombies are downstairs in the house, they can hit you even if you are upstairs. I died like this.

This occurred on Dec. 26th. My game was fully updated.