These are the ones I find the scariest - I've had two that spring to mind, one coming towards a roundabout in France (driver acknowledged his mistake by his expression when he saw me after his idiotic manoeuvre), and one on the A30 in Somerset - combined speed would have been about 85mph, and left about 2ft spare as I veered towards the side of the road. If I see someone overtaking towards me, and have time, I'll indicate/point right while staying over to the left, in the hope it might make me more visible to the idiotic/careless overtaker.