Bags on seats
I'll tell you what's always good fun... When you get onto a train which is packed full of people and some tosser has sat down and put their bag next to them so that no-one sits next to them...
The reason this is such fun is that you can walk over to the seat and begin sitting and one of two things will happen:-
1. They'll snatch their bag out of the way and look at you with evil "you bastard, you ruined my plan" eyes.
2. They'll assume you won't actually sit down.
Of course the first option is preferable as it pisses em off and avoids confrontation but if they don't move their bag I just keep sitting down until I crush whatever's in their bag with 13.5stone of Dave. :)
That's usually the point at which they shove you to get off their bag and then they pick it up and say "what the fuck are you doing" at which point the best reply is always "Sitting down on a seat which is built for people, not bags" following by you sitting down as last time.