The robot won’t merge a change in the queue if it can’t be merged or tests fail. The changeset would be left open and the developer notified to fix it.
The whole process assumes that multiple changes in the queue don’t depend on each other, if they did, it should all be in the same changeset.
It assumes most do not, but it’s entirely possible for someone to change a common library which makes several down stream changes wait. Even if there are no merge conflicts, if they effect the same tests, changes will have to wait.
The whole process assumes that multiple changes in the queue don’t depend on each other, if they did, it should all be in the same changeset.