Mark Rober has an awesome video on this which outlines a lot and gives a very high level bit of info on it. I can't imagine the amount of stress these engineers are feeling watching this delayed feed.
Well, the big day comes and guess what? The spacecraft performs perfectly. Everyone was absolutely ecstatic.
It turns out the test bed had been misconfigured, and there was nothing wrong whatsoever with the spacecraft or any of its command sequences.
Really makes you think about how to make the best possible decisions in a crisis.
If the probe is already near its destination, you have a very narrow window during which you can adjust its velocity for intercept.
If you delay too long you do not have enough deltaV to intercept, and either miss your target entirely, or end up in the wrong final orbit with no ability to correct your inclination or eccentricity to what your mission parameters call for. Then you've basically spent years to send a paperweight to Mars.
The minimum-energy launch window occurs every 2 years, and lasts a couple of weeks.
It's like the risk of losing a winning lottery ticket. Broadly the risk is negligible, but when you zoom into the precondition of having won the lottery, it's a big fucking deal.
Better computing resources, better simulations, better materials, better and cheaper process control have enabled engineers throughout the entire supply chain to do more with less and when you take snapshots of progress at multi year intervals like NASA does with its Mars missions the progress really shows.
I asked him if he was stressed, his exact reply: "nah not really, not a whole lot we can do now"
:D