What first-hand experience has terrified you the most without actually doing you any harm?
This isn’t about false alarms and imaginary fears. You didn’t make it up. Something took place outside your imagination, and it could have been bad. (And this is about you, not about fear for someone else.) It just ended up without damaging you after all.
An extreme amusement park ride? a risky adventure?
A tumor that turned out to be benign? a narrowly averted loss?
A close call in traffic? witnessing a crime?
The key here is that nothing bad actually happened to you—you came through unscathed. But you were scared half to death anyway.