A thousand years before the Hero of the Wild, where the Sheikah Tribe have just finished the first batch of their brand-new Calamity fighting robots. Location - the testing grounds, Hyrule Field:
The Leader of the Sheikah Tribe stared intently at the blueprints the head Architect of the Guardian project had just provided her. Then she looked up at the actual Guardian, standing alert and glowing blue just in front. She glanced to her left, where the head Architect stood waiting. Then back to the blueprints. Then at the Guardian again. Back down to the blueprints. Slowly, the skin on her forehead furrowed.
"Correct me if I am wrong," the Leader began, "for I am not an Architect, nor am I skilled in the ways of machines. But it seems to me that, this design appears to have a blatantly glaring weak point?"
The Architect leaned over to see the portion of the blueprint that his Leader was indicating.
"Ah, yes, that's entirely correct," he said, not at all sounding like that was a problem. "There is a weak point in that location."
His Leader gave him a pointed - and bewildered - stare. "And… you don't think that might be a problem later? This is the Calamity we're dealing with."
"Yes, that's why the weak point doubles as the laser port," the Architect explained.
"But according to these," the Leader insisted, gesturing to the blueprints, "the eye of a Guardian is the access point to literally all of the internal functions. One hit of the proper amount of power and the whole thing explodes."
"That is true, yes," the Architect agreed serenely.
For a moment, the two Sheikah stood in baffled silence.
"Why," the Leader said, pinching the bridge of her nose between her fingers, "was this the version of the Guardians that made it to production?"
"Because it was the first working version that didn't have the spontaneous self-destructing issue and we are running on a very tight schedule, ma'am," the Architect reported. His Leader squeezed her eyes shut at the reminder; the Calamity, much to everyone's growing terror, was all but scheduled to make an appearance by sometime next week, and the King of Hyrule had basically told the Sheikah that, flaws or no, the country needed a defense system now. And, to be fair, the spontaneous self-destructing issue had stubbornly refused to be solved until very, very recently.
"Okay," the Leader said, removing her hand from her eyes. "Is everything else shielded, at least?"
"The Guardians are virtually indestructible at any other spot," the Architect informed her.
The Leader let out a heavy breath through her nose. "Well then, let's hope this doesn't come back to bite us."
The Calamity ended up attacking right on schedule, and was quickly and promptly contained. Unfortunately, the ease of this feat meant that none of the Sheikah actually bothered to remember to write down the bit about the weak point in the Guardian's armor, and subsequently, when the Calamity broke free a thousand years later and exploited that same weak point to take over the entire Guardian force, the whole thing ended rather badly for the Hylians that day.
Interestingly enough, it never occurred to the Calamity that the same passage it had used to infect Guardians with Malice was also wide open for any other sort of input as well. Granted, the Calamity was a roiling mass of hate and magic and didn't have the clearest thought process, but one would think that it was just common sense to shut the door behind them. But, then again, the door shot lasers. It would be fine.
Of course, a hundred more years later, the Hero of the Wild discovered that if he shot a particular kind of arrow in this exact spot on a Guardian's head, the whole thing would explode like a TNT barrel on Death Mountain. He then proceeded to use this knowledge to storm Hyrule Castle and beat the crap out of a great deal of Corrupted machinery along the way.
(Somewhere in the afterlife, a Sheikah Leader and her Architect looked at each other awkwardly.
"When that kid gets here, you are apologizing for that mess," the Leader ordered.
"Yes ma'am," her Architect mumbled obediently.)
"Well that's a bit unfortunate," Link muttered to himself, staring at the pages of The Sheikah Conspiracy: The Real Reason why Their Technology is so Susceptible to Corruption. The author was a bit of a crackpot, and seemed to have gotten the Sheikah mixed up with the Yiga, but ironically enough his research was sound. In any case…
Link reached for his parchment.
"Rule… Number… Twenty… Five…"
Rule Number Twenty-Five: No matter how well it would perform, I will never construct any sort of machinery which is completely indestructible except for one small and virtually inaccessible vulnerable spot.