You’ve probably seen (a less crunched version of, sorry! only gif I could find) this video before, of a Russian Proton-M cargo rocket going out of control immediately after liftoff at Baikonur, pointing the flamey end towards space, and exploding in a quite spectacular (and toxic, given that the propellants were a hypergolic mix of N2O4 and UDMH, both obscenely unpleasant substances) fireball.
Among other questions, you might be asking “why does a launcher rocket in the 21st century use these fuels” and “why was there no flight termination system”? The answers to those are “Proton was originally an ICBM design for the Tsar Bomba” and “The Russians decided it’s fine because Kazakhstan is sparsely populated”. They’re also not important questions , because this whole disaster was unrelated to those facts. You want to guess what the cause was? A faulty accelerometer.
Now, you may wish to interject, “But Morgan, aren’t these things usually redundant? How does a single failed accelerometer doom a rocket?” And you’d be right. It wasn’t a single faulty accelerometer. It was three. And, to be frank, I’m being rude to the accelerometers. There was nothing wrong with them.
Now, as a quick aside, what you’ve gotta understand about these particular pieces of kit is that they only fit in one way. They also have arrows, as you can see, indicating the correct direction to install them. What investigators discovered was that all three yaw-measuring sensors had, somehow, been installed upside down, something that was assumed to be impossible.
It turns out some idiot inexperienced technician had forced the things in the wrong way, without stopping to consider once that there might be something wrong with what he was doing. When the rocket lifted off, the yaw sensors sent nonsense inputs to the flight computer that ultimately doomed the rocket.
So, in short: An inexperienced technician managed to install a critical part wrong. This wouldn’t be a shock (especially with proper oversight, where it might have been caught). The kicker is that he managed to make a mistake that was thought impossible, both physically and logically, by hammering a part that only fit in one way, and had markings showing the correct way, upside down.

