In a GitHub Actions file that is set to run like this
on: push: branches: - branch_name
The action will run when you create a pull request from branch_name to another branch.
This issue could be very bad when someone has a fork of your repository and creates a pull request from branch_name to their own fork, which would trigger the Action despite it not being a push to branch_name on origin.
I hope this is not a duplicate, I couldn’t find any reports of this issue when attempting to search for it here and on Google.