There are a million and one of these, let's go for my top three incidents:
1) filtering, legally, on the M3 on my ZX9 a car in the outside lane suddenly veered across the motorway 20 feet in front of me. Not the poor guy's fault, sometimes you can catch your broadsheet on the steering wheel when you try to turn the page;
2) whilst moving house I hired a box van, in going back for the second load I spent 3/4 mile trying to pull back in front of a TIR lorry, that accelerated to block me, when I finally managed to pull in he tailgates me for the next 1/2 mile until we needed to stop with alacrity on the M3/M25 junction, he failed to stop, pushed me into the car in front, was the cause of a major pile up (detail really not needed) and then tried to tell the police I was guilty of dangerous driving - there's a skip somewhere needing his body parts to stick out;
3) me, yup, I messed up, going up the A5 in a serious thaw,I thought I'd follow a car 1/4 mile in front and match his speed, a car pulled across in front on me in the other direction, turning right, I couldn't see the turning and tried to stop, 150 yards of skidding and I hit them at 30mph, not their fault, mine, but I thought I was behaving sensibly, guy's realise you're really dumb in your 20s