If you use CI, this has happened before.

Build Failed

Maybe you've got Travis set to build every pull request.

In which case, there will be a notification at the bottom of the pull request page that describes whether it succeeded or failed.

But it only checks each one in isolation. The merged build can still fail.

This happens when two commits make changes to different parts of the code that clash with each other, like if one adds a new call to a function that the other renames

You need to check the combination, before it goes to master.

Bors will detect, when it gets around to the second one, that it no longer applies cleanly.

That's what bors does.