Pull requests wait in line, and each is tested on top of main plus everything ahead of it before it merges. Two changes that pass alone and break together get caught before they reach main, not after.
Agents open pull requests faster than people do, so two green ones colliding stops being rare. One renames a function, the other adds a new call to it. Git sees no conflict, and main is red for everyone.
Say it to your agent
“Add merge_group to the triggers of our required CI workflow, alongside pull_request. Then tell me the exact setting I need to turn on in branch protection, since you can't do that part.”