I've seen a few good ones this week:
Numerous people in the left lane at a roundabout who go across the roundabout in a straight line, regardless of whether there might be someone they are about to collide with (me!) in the inside lane.
Going down a long, steep hill on the A30 dual carriageway in the outside lane at about 75 overtaking people in lane 1. Three cars pull onto the carriageway ahead of me doing about 60 mph, the one at the back indicates right to pull into the outside lane (with me rapidly approaching), stops indicating, and then pulls out anyway. Use your mirrors!
Someone moves into lane 2 (again on the A30), suddenly indicates left and dives across lane 1, over the ladders painted on the road just before the barrier starts, and in front of a car already on an exit slip road, just so they could take the exit they almost missed.
And one from a couple of weeks ago:
Someone in a lay-by at the side of the (you've guessed it!) A30, who decided that rather than waiting for a gap in the flow of traffic and then pulling out and accelerating up to 70, would actually just join the A30 at 20 mph regardless of what may be approaching, causing me to have to hit the brakes and swerve into lane 2 to avoid slamming into the back of them at 70 mph. Cretin!