To all of those whiners out there with complaints similar to this one: "He/She did something right in front of me that I need to bitch about as an anonymous person"
DO SOMETHING ABOUT IT! You were right there! You could have told the person to back off, for example: "Hey, that seat isn't for you!". It drives me nuts that people are soooo afraid to talk to one another that simple problems can no longer be resolved!
I'm not telling you to start a confrontation, be civil about it!!! There is nothing wrong with politely asking someone to correct a wrong! Now if they want to be a dick about it, then you can lay into them. —Annoyed