When validating the segments Magnolia seems to check if any two of selected segments share the same starting path
In SegmentationMultiField

            Node node2 = session.getNodeByIdentifier(uuid2);
            if (node1.getPath().equals(node2.getPath())) {
                return false;
            }
            if (StringUtils.startsWith(node1.getPath(), node2.getPath())) {
                return false;
            }
            if (StringUtils.startsWith(node2.getPath(), node1.getPath())) {
                return false;
            }
        }

I two last checks if the first segment is parent of the second one fail when the name of the first segment start with the name of the second segment (and vice versa):

• StringUtils.startsWith(node1.getPath(), node2.getPath())
• StringUtils.startsWith(node2.getPath(), node1.getPath())

Otherwise if one defines and select segments like:

• FOO
• FOO+BAR
  It will result in a validation error