Who do not read the fucking OP.
There is nothing more infuriating than a poster who writes their issue and explains that they cannot do A and have already tried B and then the majority of posters come along suggesting A or B. 
I also cannot stand people who come along with a know it all manner and dish out commands using really short sentences. For example: Ambulance. Now.
No doubt I ABU but there are no two things surer to give me the fucking rage.