So annoying when someone sitting up your arse even though you're doing the limit, then they overtake, only to get in front of you and then just potter along, but slower!
Or you get so fed up of them sitting in your boot you finally decide to let them overtake, but then they immediately take the next junction/turning.
Strangely enough though, the ones that insist on tailgating when your sat dead on the speed limit never seam to want to overtake when given the opportunity just before a big yellow fixed speed camera...