Nothing instantly comes to mind as completely terrifying (I don't scare easily at the time, more after the fact when I go over it in my mind), but 2 things stick in my head.
The first one happened this Sunday night. Was doing my usual late night drive to Leeds from Liverpool after spending the weekend with Lynnie. I've driven it hundreds of times, so know the roads like the back of my hand. Came off the M62 at Junc 27 onto the M621, and there's a long sweeping right hand bend. Now I now exactly how fast I can take the bend, and was going at that speed. Unfortunately however, I misjudged the road conditions which were very slippy and greasy. My back end stepped out, and instead of doing the right thing (coming off the gas and counter steering), I did the wrong thing (held the turn and touched the brakes to slow down). Cue my back end whipping completely out, going sideways onto the grass verge, and ending up with my car pointing the way I'd just come from. I remember not being scared at all while it was happening, instead watching very calmly where I was going and waiting for the car to stop. I imagine if I'd seen a wall or tree in my path, my reaction may have been somewhat different. Thankfully there were no other cars around (it was 3am), and I was completely injury free (bar my pride). The car was a slightly different story. Both front tyres need replacing, and a suspension arm is bent. I think I've gotten off lightly considering what could have happened had someone been around, or I'd hit something solid.
The other scary thing wasn't scary at all when it happened. When playing cricket a couple of years ago, I was batting and took a ball to the head, on my temple just above my right eye. This split my head open slightly, and knocked my silly for a few moments. The ambulance came and I went to hospital to get it checked out. I was fine in the end, and had the cut glued back together again. What scared me afterwards however, was thinking about what would have happened had the ball hit me elsewhere. 1 inch lower and I'd have broken my eye socket. 1 inch lower than that, and I've broken my cheekbone. 2 inches lower than that, and I've probably broken my jaw. I was always a bit more wary when playing cricket after that, though I never let it stop me. I just duck faster now