Police, Teachers and pretty much anyone with some sort of authority over a lot of people/kids. Every time a Police man stops me it always ends up in an argument and him saying 'Did you study law for 3 years?' and my reply is always 'No, but there is a website called legalisation.gov.uk which means I don't have to. Most policemen don't even know the laws themselves and quite a few make them up as they go along. Especially when dealing with adolescents minding their own business.
Also annoys me that they can't have any fun even when stopping you for no reason at all.