I think it is normal to have a few slip ups here and there while they are still puppies, I have found that the accidents we had, either occured because the dog was in a different room such as being in the living room, when normally out in family room will go to patio door and scratch with paw...or if excited, will just "forget" and go.
You can usually tell when they start sniffing, it's time to go. You have to keep such a sharp eye on them all the time and it seemed like the minute I turned my head, he would start going. I would say if it is happening sporadically it is okay. Something happening daily could be a sign of a medical problem.. :(
Good luck. :)