Well, with first-generation GMOs, a single gene was inserted into a crop. This is totally unlike all previous crop breeding, where changes were complex and no s

Well, with first-generation GMOs, a single gene was inserted into a crop. This is totally unlike all previous crop breeding, where changes were complex and no single gene was likely to control the trait. So before GMOs, there was basically no chance that a trait bred into crops could escape into the wild by gene flow, but with first-gen GMOs that became a real possibility. It has indeed been recorded happening a bunch of times, leading to, for example, herbicide resistance in weeds. I should point out that herbicide resistance in weeds was widespread before GMOs by other mechanisms, and in fact the switch to glyphosate has been a great thing because we've massively reduced the application of harmful chemicals, while glyphosate is benign. But the fact remains that GMOs do carry their own specific risks that are a result of the technology and methodology, rather than the specific application.