A blink and you'll miss it example. As the sub is filling with water, Audrey slams the door on at least two fellow engineers.
Fridge Brilliance: The whole sub was filling up, without that engine door closed, more of the sub would be flooded, and more lives would be lost.
Or what about the friend Packard was speaking with over her intercom (the one whose husband left her)? Well, the chances are high that she was on the ship as well. There were no female survivors other than Packard, Audrey and Helga. Understandably, Packard looked quite disturbed after the moment of mourning the dead.
Or the fact that the Atlantis they found was only a small portion of the whole continent.
Most of the Atlanteans are most likely hundreds of years old, and it's heavily implied that this long age is granted to them via the giant crystal under their city. It's certainly proven that the crystal is all that keeps them alive. Now, if Rourke had succeeded in stealing it, what exactly would have happened? Everyone would have aged to dust? Or would everyone have just fallen over dead?
Maybe they'll just start aging normally from that point? Still Fridge Horror since because they normally life for a very long time, they're not prepared to have children at a population sustaining rate for they're new lifespans, including at best a 40 year window for the women.
Pay very close attention to the scene when Rourke throws Helga off his blimp. As Helga is being tossed over the gondola's railing, you can see her fall in the direction of the blimp's propellers, which are located just below the gondola. She misses them, but it's not hard to imagine what might've happened otherwise.
Milo's surface life is effectively ended before he leaves on the mission: his books are in storage, his clothes are packed, and his resignation has been sent. It's implied he has little to no relationships, so that's fine, too. But he never sees his cat again.
Either Audrey or Whitmore adopts the cat. It's sitting on her lap in the last scene in Whitmore's house.
Before he dies, the King of Atlantis gravely warns Rourke and his crew about taking the (very angry)mother crystal. We find out why when Rourke gets cut by a crystal shard. Had Kida/the Crystal reached the surface, countless others might have suffered this fate.
When the toddler Kida loses her toy at the beginning of the movie, as they flee the incoming destruction, her mother says not to go back to get it because there's no time. She kneels on the ground, calmly and slowly explains this to her daughter, and takes several more seconds than it would have taken for Kida to actually dash back and grab her toy.
Similar to the above: when the Leviathan cripples the Ulysses and Helga orders to sound the alarm, Packard takes her sweet time actually obeying her. How much faster could they have gotten out of the ship and out of range of the Leviathan, and how many more lives could they have saved, if she had complied immediately?
Probably, zero more survivors. The time wasted amounted to seconds, and no one else was slowed down by her, except maybe Helga. Honestly, I can't imagine anything happening differently had she left her station immediately.
Generally knowledge becomes lost because those who had it die out without passing it on. The Atlanteans are effectively immortal. What, everyone forgot how to read? Why?
Wrath of the Gods?
Probably out of fear of the same knowledge and technology that destroyed their civilizations. Likely the King suppressed all forms of knowledge for fear that some upstart could discover the secret of the Heart of Atlantis and cause yet another disaster. Therefore, the Atlantis that we see is losing all knowledge of its own cultural identity, its people are kept blissfully ignorant of their own cultural identity, and very likely have forgotten, or never even learnt, how to read in the nearly 9000 years since the flood.