There's a saying in Civilization: "There are three hard problems in system design: Robustness against intelligent optimization, generalizing 100% correctly to a never-previously-tested problem on the first try, and maintaining the consistency of a Network-distributed data structure." The joke/warning being that the third problem is allegedly that hard; the difficulty of the first two problems is considered obvious.
Hard problems are hard, often, because they have subtleties; gotchas that zap you when you wouldn't first-order expect them.
Of the first hard problem, a gotcha is that things blow up in the face of intelligent optimization even when those intelligences are not adversaries. When you run a prediction market, the trading parties aren't adversaries, per se, but they want money out of your system and will take weird execution paths to get it, if any such execution path is available. Even the programs the traders run, if they're executing simple means-end searches for maximum extractible value, can be considered as a kind of intelligence within the system against which the system must be robust.
Of the second hard problem, correctly generalizing out-of-distribution with 100% accuracy on a first try, a famous subtlety is that Reality has been known to be really ridiculously strict about what counts as a 'new and untested domain'. Reality will kick you over the cliffside without hesitation even if you think you're in a domain very similar to a past domain, if you didn't test exactly that set of conditions.
And of course while it's sometimes hard to generalize correctly to a new domain at all, it's often a lot easier to do it right the second time, or if it's okay to get a few outputs wrong here and there.
Civilization has a lot of medical prediction markets. It has a lot a lot of medical prediction markets. Most medical markets don't experience this kind of attention, sure, but some do; it's not unprecedented for a rich patient to bid a million labor-hours on their outcomes. Civilization has run thousands of medical prediction markets that saw higher bids than Merrin's patient, more trading, or more complicated and weirder medical problems. If it was just that, the problem would be as 'inside of a previously tested domain' as any unprecedented complex event inside reality ever gets.
Few of Civilization's medical prediction markets are secret prediction markets, however. There are routine secret prediction markets, some with a lot of liquidity, but those routinely secret markets are not medical markets for complicated medical conditions with lots of liquidity and traffic.
The case with Merrin's patient is, in fact, the very first time that a secret market operated by Exception Handling, in the medical domain, has dealt with a case that had this much liquidity, this much trading, these many medical complications... and, now, a particular malfunctioning sensor.