For me it is when my tutor wipes the pen off the board and misses a bit in the corner.Or when it has been sunny all day and just when you want to go for a walk it rains but worst of all is when you rush home to give you pomeranian a hug and it runs straight away from you to your Dad!!!Grrrrr.
People eating noisily at their desks - crisps, apples, carrots (all munch munch crunch crunch rustle rustle etc etc grrrrr) or someone stirring their mug of tea/coffee 20 million times (ding ding arghhhh). Deep breaths now messie, deep calming breaths ...... stay cool.
I totally agree with you about the wiping the marker pen off the board. That used to irritate me when I was in my maths lessons and she would always miss a bit and I just felt like standing up and wiping the board clean!
Other stuff that irritates me:
People who take ages to get their money/card out when paying for their shopping.
People taking long to take their order at a fast food restaurant for example, when they had all that time to think about it while queuing.
Like others, people who don't indicate when driving and i'm sure there's more but i'll stop lol.