I think, in some (not most) circumstances, there are "oops" or accidental litters.
My grandpa had a cattle dog. When she came into her first heat, he locked her in the shed when the other dogs when out to work. A neighbor's bulldog literally CHEWED through the wall into the shed and bred her.
So, in some extreme circumstances, I do believe that there are accidental litters. I believe they're seldom purebred, though. That's too convenient.
Gable: Black and Red GSD: 03/02/2012 -
Suki: Russian Blue: 03/02/2013 -
Sully: Tri-Color Collie: 04/22/2016 -