In that case, I’m glad I didn’t say it was a hard and fast rule, with no exceptions. ;)
You did send me to Bing, though, were I found this nugget, fwiw:
“If you want to adopt the healthiest, happiest puppy possible, wait until it is nine weeks old to separate it from its mother and littermates by bringing it into your home. At this age, you can be relatively sure that bite inhibition and basic housetraining habits have been learned. The critical socialization period for puppies lasts until the age of sixteen weeks, giving a nine-week-old puppy plenty of time for proper socialization in its new home.”
http://petlvr.com/blog/2009/09/02/best-age-to-adopt-a-puppy/
I’ve taken two dogs at nine weeks. Both are sweet, but both have problems, having been in less than optimal environments. I think that in many, if not most cases, 9 weeks is a good time to bring in a new puppy.