Add to my previous list.
RTS Gamers who constantly spam superweapons (examples of which are the CnC: Generals Nukes, Particle Cannons, and SCUD Storms) then call out hax when I destroy even a single powerplant, or their entire power grid with my airstrikes.
And those who constantly tank (especially Overlord, Apocalpyse, Paladin, Battle Fortress and so on so forth.) rush, I mean seriously, it generates lag (especially in Generals), and is a n00bish tactic. Thankfully they're easily countered by a Nuke or SCUD strike, air strike, or some cleverly placed mines and turrets in a key chokepoint.
And people, stop generalizing that 13 year olds and below are some of the worst breeds of gamers. It's insulting to me, I'm 13 for Rizal's sake!