Also, cyclists going two abreast along a road when there is a car waiting behind.
I was on a run one day and I saw this in front of me. The car could have had massive spikes on the front ready to impale the cyclists, but the cyclists weren't moving for anyone. Tossers.
Oh yes, and people cycling on the A82 during the day when the road is at its busiest. Always makes me cringe. This might not sound like much, but due to the local geography, the A82 IS the main road, so, although it isn't a motorway, it might as well be at certain places!