There are plenty of TechNet articles, like this one that say that phantom object don't get updated if an Infrastructure Master is also a Global Catalog, but other than that there isn't a lot of in depth information on what actually happens in this configuration.

Imagine a configuration like this:

|--------------|
| example.com  |
|              |
| dedicated IM |
|--------------|
    |
    |
    |
|-------------------|
| child.example.com |
|                   |
|  IM on a GC       |
|-------------------|

Where child has two DCs that are both global catalogs, meaning that the Infrastructure Master role is on a GC. And, example has three DCs with the Infrastructure Master role on a DC that is not a GC.

I understand that it's usually best to just make everything a GC and not have to worry about this sort of thing, but assuming that's not the case - what is the exact error behavior that can be expected from a setup like this, and which domain(s) would this behavior manifest in? The child or the parent?