It can be done by accident. Someone in one of the flats near me has an unprotedted WiFi connection, and my laptop accidentally connected to it even when my router was on: they were in the wrong order in the network listing.
I only discovered this when my anti virus update went at about 1/5th the speed I expected and I then discovered the update had been dragged through someone elses router!
I have since told this person how to protect themselves!
B