Mine were a bit less acute than most cases here. It doesn't rank up to the kind of emotional trauma other people ITT have been through (though I'm not a complete stranger to that, either) but mine was when I realized my health is going to prevent me from ever doing what I want to and getting my shot at a reasonably happy life.
The slow dread of realizing decades of miserable, exhausting, bitter, mostly hopeless, unappreciated effort is void, and has been a complete waste - realizing that things "working out" is not really on the table anymore, and neither's anything else, much: all you can do is keep existing. That is easily #1.
Or realizing friends and family didn't have my back the way I thought and might actually join my list of a zillion problems. That was pretty scary.
Distant third, near misses in traffic - but frankly, I've had a stronger reaction from losing my fucking house keys. Almost got hit by a tram. Meh. Would've lost my appointment, I'm sure. Some shitheel trash in a BMW (because of course it was) tried scaring me by pretending to hit me while out for a walk, I'm like "😐 ... yeah? Make my day - in fact, throw it in reverse and get a proper run-up, you little bitch".