An un-mated female corn may lay infertile eggs (slugs).
I have had this happen several times, but it was usually a first-time breeder that laid a second clutch when I didn't want her to.
I did have a 2 yr old that wasn't brumated lay a small clutch of slugs.
And on the other hand, I had a proven breeder (several times over) that simply refused to mate last year and didn't even produce slugs. Go figure!