Ridcully refers to Shawn's supposed father, Sobriety Ogg as someone he knew when he was in Lancre as a boy. Then he starts talking about witches, before stopping himself when Shawn says "Our mam's a witch". The problem: It's firmly established in Thief of Time and The Companion that witches, and Oggs in particular, are matrilinear. So Sobriety Ogg would only be so called after he married Nanny, which not only doesn't fit the timeframe, but means Ridcully must know Shawn's mam's a witch.
The regional language is called Oggham. Nanny points out that the Oggs have been in the Ramtops for a very long time. It's not impossible that Sobriety Ogg was already so named long before he married Gytha.
Or Ridcully assumed that Gytha Ogg took her husbands name, and had no idea it was the other way about. Nanny may never had revealed she was a witch. Doesn't resolve the timeline issue however.
Actually, Shawn says his mum's a witch before the name of his father comes up in the conversation. And Ridcully doesn't specifically say he was a boy when he knew Sobriety Ogg, so he might well have met the man after he'd gotten married.
Whatever happened to Ponder's career as Verence's Court Wise Man?
The same thing that happened to his career as the God of Evolution's assistant, probably. Would you stick around Lancre once you realized that Granny Weatherwax would be looking over your shoulder all the time, saying she "can't be havin' with that", every time you try to do some arcane research?
Or worse, telling you answers that are obviously correct, but not at all what you were looking for.