Driving my best friend to the hospital, when he was in mysterious & apparently excruciating pain. I can't even remember what he had -- some failure of one of those essential organs we don't think about much. It was a side effect of the medication he was on, but when I looked up the syndrome, waiting in the hospital, I found out it was kind of mostly lethal.
(Spoiler alert; When it's a side effect & not the result of essentially drinking yourself to death, it's a lot easier to recover.)
But I literally thought that my best friend in the world, who is also my boss & the father of the child I am a quasiparental towards, had less than a 50% chance of survival. I was making contingency plans in my head, trying to figure out how he & my other dear housemate-friend could possibly afford to raise the kid, and how I could even function in any way, if we lost him.
...I'm lying, actually. There was a worse time. But that one, I think, is not my story to tell. I'll just say that the stronger your love for someone, the more afraid for them, & for you, it's possible to be. It helps, though, if you are not alone in your fear.