My “big one” was a jacknife, and it happened a week after I started driving Class 1 (I’d been on rigids for a year first).
I was driving an old 2 series Scania with no speed limiter, pulling an empty Euroliner south on the M40 at about 3pm on a sunny, dry afternoon at about 55mph - our preferred running speed to keep the fuel bills down, although we did boot it if we were tight for time, which fortunately on this occasion, I wasn’t.
I was between junctions 11 and 12 when suddenly and without warning the brakes came on. All of them. Completely.
I only have a dim recollection of the event after that. I remember the world going by from left to right as I looked through the windscreen. I remember an almighty crunch as the unit hit the embankment. I remember a bloke called Robin forcing the cab door and I remember saying over and over “What’s Mick going to say?” (The subbie I was driving for).
Piecing things together afterwards, I apparently tried to steer straight, and evidently succeeded for a short while as the skid marks went straight for 100 yards and everyone else had time to get out of the way, so no other vehicles were involved, despite the fact I ended up blocking 2 lanes of the motorway, with my arse end hanging over the third. I take no credit for that, though, as it was instinct rather than judgement. The Police told me later that had I been going just a couple of MPH faster, or been driving something built less like a tank, I wouldn’t have survived. As it was, I escaped with severe bruising down my right side where I slammed into the door so hard there was a Lucy-shaped dent in it, and a bump on my head which remains to this day where the CB flew out and landed on me. Again, to this day I have a bizarre and irrational phobia of having a CB fitted in a lorry. Daft, but there we go. I’ve tried to get past it but to no avail, the CB was last seen in the back of my Landrover, after I chickened out of having it fitted yet again.
The bottom 2/3rds of the wagon were basically destroyed on impact - the o/s from hitting the embankment, and the n/s where the trailer slammed into it. The trailer was practically brand new. Since I was working for a subbie at the time, no trailer meant no job, but by coincidence I’d had a voicemail on my phone from a local tanker firm I’d applied for previously, offering me an interview. 24 hours later I had a new job, and a week after that I drove over my own skidmarks in a Taymix wagon. That nearly didn’t happen as I bottled it the first couple of times I went out after the crash, but they were understanding souls there, for all their faults, and bore with me until I pulled myself together. (Bob the Dog take note. 
)
To this day we don’t know the cause of what happened. VOSA found nothing wrong with the truck or trailer. Further investigation over the next few years, including talking to other drivers who had suffered the same experience, leads me to believe it was likely to have been a handbrake valve fault, but this has never been proven conclusively as the unit had been written off and scrapped by then. Both the owner and I wondered for a while whether I’d put my foot on the brake by accident, as I was wearing seriously chunky boots at the time, but I tried to reproduce the event a month later on Filton airfield (in a Taymix wagon, apologies to Rory if he reads this, 'cos I never asked or told him what I’d done with his truck… 
) and it couldn’t be done, you can’t get that kind of braking force with your foot, it had to have been a catastrophic failure of some kind. The tacho from that day shows a vertical line where the brakes locked up all at once, with maximum pressure. So whatever it really was, it definately wasn’t driver error.
It’s not an experience I would recommend to anyone, although I do believe I’m now a safer driver for having seen exactly what these vehicles can do. The whole incident gave me a very healthy respect, if nothing else.
Ps. If your name is Robin, and you were driving the car transporter directly behind me when this happened in 2001, then I never got chance to properly thank you. So I’ll say it now. Thanks.
. keep the pocket knife tales coming tho cuz it still amazes me what some people have experienced over the years and still around to tell the tale. stay safe 