Guitierrez made two assumptions based on the memo. (1.4.18)
If there's a lesson to be found in Jurassic Park—besides, you know, don't clone dinosaurs—it's don't make assumptions. A lot of trouble is caused because characters act on their assumptions instead of fully investigating the situation. Their overconfidence inevitably leads to dire consequences.
"There is absolutely no problem with the island." (2.10.37)
And the Titanic was "unsinkable." John Hammond is not being hyperbolic here: he really believes that none of the issues with the island amount to a problem. His refusal to see reason, of course, eventually results in his own death.
But if planting deadly ferns at poolside was any indication, then it was clear that the designers at Jurassic Park had not been as careful as they should have been. (3.17.30)
Ellie Sattler notices that poisonous ferns have been planted in an easily accessible area. If the park were to open, then children would surely touch them, getting sick or worse. If the designers of the park can't even get ferns right, chances are their designs for keeping the dinosaurs under control have T.-rex-sized flaws.
"I wouldn't call them tremendously intelligent," Malcolm said. (3.20.97)
Malcolm, usually more astute, downplays the intelligence of the Velociraptors after they try to attack through an electric fence. Malcolm is usually right in his predictions, but his assessment here turns out to be wrong. The raptors are wicked smart.
"So I wonder: have they learned, somewhere along the line, that humans are easy to kill?" (3.20.127)
Malcolm has put on his Sherlock hat. If the raptors behave as though humans are easy to kill, then they had to learn that lesson from somewhere, and that means that the raptors have killed before. Hammond hasn't been honest with them about everything, now, has he?
From the beginning, this had been one of the core beliefs of the planners. The animals, however exotic, would fundamentally behave like animals in zoos anywhere. (3.24.25)
This is another assumption that proves disastrous. If they'd thought about their plans, they might have realized that recreated animals from millions of years in the past might not behave in the ways they expect. Even paleontologists, who devote their lives to studying dinosaurs, aren't so sure how dinosaurs actually behaved.
Nothing as basic, or as serious, as the possibility of an animal escaping. Wu found it offensive to think that anyone would believe him capable of contributing to a system where such a thing could happen. (3.26.9)
Wu has way too much faith in his own intelligence, and he suffers from a bad case of the bad kind of pride. He refuses to accept that the animals are breeding—and that he made a mistake—even when the evidence is conclusive.
"If they don't do it, someone else will." (5.45.124)
Malcolm doesn't think much of scientists, particularly those who act without any ethical restraint. He believes scientists justify their problematic work on the basis of the assumption that someone will do it eventually, anyway, so they might as well do it first.
And when the lights and screens came back on in the control room, it never occurred to him that main power hadn't also been restored. (5.47.46)
Arnold has his own deadly assumptions. He believes the park is fundamentally sound and that, whatever minor disasters arise, they won't take away from his control over the park. As a result, he doesn't verify that everything is as it ought to be. When he restores power to the park, he doesn't check to see that it's the main power running everything. Turns out it's the auxiliary power, and that leads to some massive problems.
"Your doctor Wu does not even know the names of the things he is creating. He cannot be bothered with such details as what the thing is called, let alone what it is." (5.47.99)
Malcolm points out the grievous limits of Wu's genetic engineering. He simply doesn't understand the kinds of things he's making. He doesn't even understand the basic biology of the animals whose DNA he uses to fill in the blanks. As a result, he makes dinosaurs that can switch their sex and therefore breed. Oops.