If you make a mistake fuelling your car then perhaps it catches fire and if you have expensive car maybe this costs you $100,000. If you make a mistake fuelling JWST then perhaps it catches fire and this costs you $10,000,000,000. Enough to buy 100,000 new expensive cars. So you do it rather carefully.
Also both fuel and oxidiser are very toxic and corrosive and very very nasty. And if they come into contact with each other they will explode. So you do it rather carefully.
Biting the hand that feeds IT
