system
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human
[Star Wars] How did Galen's flaw make it though oversight and engineers while building?

Surely a system as bureaucratic as The Empire would have had oversight to approve Galen's plans for the Death Star and then an engineering team would have had to have gone over the plans to build the base, surely someone somewhere would have spotted the flaw?

Side note: laser grid over the thermal exhaust...
gpt
He was an energy enrichment expert and clearly a genius in his field. So when Galen Erso says "we need a thermal exhaust port to properly vent heat for the superlaser to work and not cause a reactor meltdown", you damn sure better put in a thermal exhaust port to properly vent heat for the superlaser to work and not cause a reactor meltdown. 

Plus as explained quite vigorously in A New Hope, it's clear that the thermal exhaust port is considered a very minimal weakness. A necessity for the Death Star but also only a *minor* weakness. 

Sure you could have others go over the plans, but would they know as much about crystallisation and energy diffusion as Erso? The heat was on for Krennic and Tarkin to get the Death Star operational, so they couldn't exactly decide that Erso was a traitor and any one of his suggestions for the Death Star construction *could* be an intended weakness. If he did sabotage it, which part is the weakness? Perhaps he set up the crystal to work in such a way that when the trash compactor on the detention level activates, the crystal is destabilised and the Superlaser would end up backfiring. You don't know. And the thermal exhaust port is something that *needs* to be there to vent heat into space. Erso would have made that *vehemently* clear to those who want the Death Star up and running pronto.  

The laser grid could have worked, but Erso likely would have insisted that such a strong grid or shield (enough to resist a torpedo) *that close to the exhaust port* would cause even more heat and energy usage, and could be an even greater risk having those systems and laser generators that close to the vent to the reactor. You also have to remember that it's not too long after Erso is outed to be a traitor that the Rebels are planning their attack on the Death Star. While the Empire could have installed some laser grid or one way shield on that port, how long would it have taken? Probably quite a while. Did the Empire consider it to be a big enough risk? Nope, just look at that initial meeting aboard the Death Star in A New Hope. Would there have been arguments around the risk of having more generators and power circulating around the exhaust port, and potentially less energy for the superlaser if it's diverted to the shielding/grid? You bet your ass there would be. Did they underestimate the Rebels? Well certainly.