I don't know that I've had one. All that caused the adrenalin to flow were things that had me going like a little kid saying "do it again dad, do it again!" I didn't do them again though, but instead of scary I found them exhilarating.
Even the times I fell it was just get up, dust off, get back on a go...... eh, maybe walk with a sore hip and shoulder for a few days but not scary.
I suppose the only briefly scary things might be thinking a rattle snake was a stick and then having it strike toward me as I whizzed by.