Another recent story told to me. A woman was riding in an aisle seat but taking the liberty of keeping her legs in the aisle. As others were getting ready to get off, the train took one of its signature halting stops, throwing passengers around like jumping beans. Of course that led to someone landing on aisle-encroacher's feet, which happened to be in open-toed shoes. A loud "OH #$*@#" was heard by all.
Moral of the story? If you get one of those aisle seats you should probably stay within the seat as much as you can. (I know, sometimes your seatmate takes over.) Aisle-encroachment can lead to stomped toes, knocked elbows, and tripping passengers. And if you're standing, be ready for the hard stops metro is known for and try not to step on any toes.