Ha ha, this thread is going to be bad for me, I can just tell.
Okay, I was reminded of this one today: people who take up more space than they need to in public areas - for example, two people walking along the middle of a path that could take three if they scooted up just a tad, and then your every attempt to say excuse me or scoot around is completely ignored. Drives me batty! One step to the side and there would be no problem!
I encountered a new version of this today - I was getting lunch, and two people were standing in front of the chiller cabinet in such a way that it was completely impossible to even see the cabinet, let alone reach it or its contents, and they seemed determined to read the ingredients list of every single sandwich before they chose anything. All attempts to get to the food were pointedly ignored. Fortunately I'm a creature of habit and knew what I wanted so I waited until they dropped their guard and grabbed my sandwich!