You know who they are. They're the first ones there, wherever there is. School, Church, some kind of sporting event. They get there before everyone else and then decide to sit right next to the aisle, requiring everyone else to have to squeeze to get through to the rest of the seats. Then they have the nerve to act put out that someone is rude enough to squeeze past them, even though we really have no other options.
In some situations this can be a huge hassle. In a whole bunch of my Computer Science courses we had aisle sitters, so finally I decided that any time I had to squeeze past one of these people, I was going to not-so-accidentally bump them a good one. Its not really in my nature and I don't recall how long I kept it up, but certainly I did it on several ocassions and always it felt good.
Another question that was raised in one of the "Letters from a Nut" books related to this topic is "What is the proper way to slide past these people?" Do you put your butt or your crotch in their face? There's really no nice way to put it.
So for future reference, if you are the first one there, do everyone a favor and slide in to the middle of the row, or if not, stand up and set out of the aisle when people are entering.