OK I've finally got round to posting this one, (it's quite lengthy)....
The single scariest moment I've had when driving was about 4-5 years ago. Heading north along the A627(M) nr Oldham, I'm in L1 approaching an oncoming slip road, (this road is a DC btw), and travelling at about 70mph. I notice a car heading down the slip road and my judgement of his position and speed suggested he was going to hit the main carriageway at the same spot as me. It was a fairly empty road so I pulled over to L2 to let him on.
Sure enough he got on, about a car's length in front of me. I'd just started to accelerate past him when for some inexplicable reason he started to cut across into L2 - when I was next to him

.
What happened next is a bit of a blur but my snap decision was that if I brake I'd hit him*, (I was accelerating at this point and was virtually parallel with him). So the alternative I took was to veer right, towards the central reservation barrier - by a massive stroke of luck there was a couple of feet of grass before the barrier at this point, although still a kerb of a few inches. I recall whacking the kerb and travelling along the grass a short distance, then glancing in the mirror to see debris flying about, (by this point I'd more or less passed the guy). Brought the car back onto the road, and just remember thinking "I'm OK, the car is still going, I'm in the clear now"
Anyway, we both pulled over to the hard shoulder and although I was a bit shaken at this point I couldn't help but notice his guilty look as he got out his car. And it turned out the debris I saw was his wing mirror being ripped off (he just caught the passenger door of my car). We sorted it all out amicably, but I think the cause was his failure to check his over-the-shoulder blindspot - I must have just hit it perfectly for him not to notice. Ever since this incident though, I've been overly-paranoid about blind-spot checking when switching lanes. To be honest I think I over-do it out of habit on occasions and take a bit too much attention away from the road in front. Just something I need to work on I guess.
* In restrospect braking and sounding the horn may have had a better result. I just don't know. At the time I'd only held my license for four years, but if the same thing happened today I may have been able to totally avoid any incident. Certainly was a learning experience as well as probably the most frightening 5 seconds of my life.