Child builds that are marked as "keep forever" get deleted along with their parent build, if the parent build is not marked as "keep forever" and "Discard Old Builds" is enabled.

Steps to reproduce:

1. Create a multi-configuration project
2. Enable "Discard old builds" and enter 3 for "Max # builds to keep".
3. Add a "User-defined Axis" called "test" with values "a b c".
4. Run the build.
5. Go to /job/<job name>/1/test=c
6. Click "keep this build forever"
7. Run the build a further 3 times.

You'll now see that build #3, along with the 'c' configuration that was marked as "keep forever" have been deleted.

Both the 'c' configuration and, I guess, it's parent build should not be deleted by the "discard old builds" process.