One job I was at had four of us working in one room. A guy from a completely different department would wander in with his cup of whatever and talk at us for at least an hour. He was both dodging actual work and interrupting us. Since he didn't report to our manager there was nothing we could do. His boss was a piece of work as well.

It's these useless people that make open plans so toxic. It's bad enough that the general noise and visual distraction decreases your ability to concentrate and get anything done, but the wandering trolls of gossip and sports talk just add an extra booger icing on the shit cake.