Fair enough. Perhaps being a relatively small mother and a large clutch, she sensed that she wouldn't be able to incubate that many eggs and deliberately excluded it.Pat I am afraid I have to disagree. From previous experiences, they don't seem to discard slugs. In this particular clutch, there was one slug and that was included in the cluster, yet a good looking egg was pushed out, or she simply didn't manage to scoop it up. Time will tell if this discarded egg is fertile or not (I didn't candle them).
Maybe she sensed that there was something abnormal about that particular egg, like it's an albino or something.