Somewhere past mile 160 on the Tahoe Rim Trail, at roughly four in the morning, I saw a handful of Skittles sitting in the dirt beside the trail.
Not a vague candy-shaped smudge. Not a maybe. Skittles. Rendered at full resolution, correct in color and count, the red one slightly forward of the others, sitting in a small depression in the pine duff like someone had set them down for me on purpose. I remember thinking, with total calm, that this was a nice thing for someone to have done.
They were berries. Sketchy forest berries, growing on a plant, doing what berries do, which is exist without candy coating and occasionally kill people.
I have spent two years standing in front of rooms full of my colleagues explaining that machines hallucinate under load. Then I went to Nevada and hallucinated my way around a lake.
The race was the Tahoe 200. Two hundred miles, a full loop of Lake Tahoe on the Rim Trail, about thirty-two thousand feet of climbing, and a hundred and five hour cutoff. I finished it. I accepted the calendar hold in February, four months out, which is exactly the sort of thing you do in February when the version of you making decisions is warm, rested, and will not be the one doing any of the running. I drove out from Grand Rapids in the Rivian. My benchmark going in was Bighorn, a hundred miles, which I ran in 31.3 hours. Tahoe was twice that distance and, functionally, an entirely different sport.
The only written account of the race I ever produced was an email to two friends. Here it is, more or less whole:
I also survived a not-so-brief 200-mile loop around Lake Tahoe. A handful of pesky sleep-deprived hallucinations tried to lure me into eating some Skittles that were actually sketchy forest berries, then another had me a moment away from scrambling down a small canyon for some water that turned out to be a pile-o-tree roots. Turns out sleep is essential to human cognitive function, in addition to being otherwise enjoyable. Who would have known!
That is it. That is the race report. Two hundred miles and my entire documentary output is a joke about Skittles.
The canyon one was worse than the berries, honestly, and I have thought about it more. I was thirsty in the specific way you get thirsty on day three, where thirst stops being a sensation and becomes a kind of narrative pressure. And I looked down and to the left and there was water. Moving water, over rock, catching what little light there was. I had already started to shift my weight downhill toward it before something in the reptile floor of my brain filed a mild objection.
Tree roots. A pile of them. Not water-adjacent. Not water-shaped, even, not really, once you actually looked.
Here is the part that interests me, and I am going to say it once and then let it go, because I promised myself this would not turn into a keynote.
My brain, under compute constraint, did not fail loudly. It did not go blank. It did not throw an error, or stall, or announce that available cognitive resources had dropped below the threshold required for accurate perception. That would have been useful. That would have been a system I could work with.
Instead it confidently produced a plausible, well-formed, fully-rendered image of exactly the thing I most wanted to find.
Water when I was thirsty. Candy when I was depleted. Not random noise. Wish-shaped output, delivered with complete internal confidence, in a format my own verification systems were built to trust.
The bad answer is not the garbled one. It is the fluent one that matches your hope. That is the failure mode I teach people to watch for, and I still walked straight into it. And I could not catch it in myself, in real time, with two years of professional practice in exactly this. I did not spot the berries because I was vigilant. I have no evidence that I was vigilant. I have quite a lot of evidence that I was not.
What actually saved me was infrastructure. A crew. A pacer walking behind me who had slept. And a plan I had written down while rested, when I was still a person capable of writing things down.
The verification habit has to be built into the system before the system degrades, because the degraded system cannot verify itself. That is true of a model and it is true of a man at mile 160 at four in the morning. A depleted brain grading its own perception is a model grading its own output, and neither one has the parts required for the job.
I should confess the actual sin here, which was not the hallucinating. The hallucinating was a downstream symptom. The sin was upstream, at Brockway Summit, around mile 149, where I had planned a six-hour sleep and took under an hour instead.
I did the math on the trail, which is its own kind of warning sign. My reasoning was that cutting the sleep would move my finish roughly five hours earlier. And I was right. The math was correct. I want that on the record, because it is the least defensible part: I was not confused, I made a clear-eyed decision with accurate arithmetic, and I traded a functioning brain for five hours on a clock nobody was watching.
Correct about the math. Wrong about the berries.
Two weeks before the race, I declined a work meeting during race week and wrote in the note: “I will hopefully be finishing a 200-mile trail run around Lake Tahoe, so will likely not be in a good place to talk enterprise AI (or maybe in the best possible place to do so?).”
I meant it as a joke. It turned out to be the most accurate thing I said all year.
Ten days after crossing the finish line, I got on a plane to the UK to run a two-day workshop for fifty-plus sales leaders, talking about, among other things, how to tell when a system is producing confident nonsense. My boss had sent a note before I left: “Enjoy that long swim. But not today or tomorrow, that is for your recovery.” I did enjoy it. My quads had opinions about the flight.
I have not decided whether I would take the six hours next time. I would like to tell you I have learned something clean here, but every ultrarunner I know has a version of this story and none of us have stopped. Somewhere out there is a February version of me accepting another calendar hold, warm and rested and entirely certain that this time he will sleep.