But a client entering a couple extra zeroes is not a clean and obvious mistake. A person could reasonably bet either $5 or $500 for example.
On the other hand, offering something like a 1000x payout for an event that has a 50% chance of happening is a clear and obvious error.
If an error is egregious and obvious, it seems reasonable that a company (in this case FanDuel) not honor the deal.
Similarly, say a grocery store meant to have a "Buy 1 get 2 free" steak sale but accidentally keyed it in as a "Buy 1 get 200 free" sale. Nobody would expect the grocery store to actually give away 200 free steaks to every customer who bought a single steak.