I always assumed that it might have been a slip up on Charlottes part. But more likely she didn't slip up to Lady Catherine because somehow I can't picture it. I think it was more a general observation to her husband who would of course feel it his duty to tell his patroness. I always wondered also if the visiting with Georgianna at Pemberly would have had the servants talking, obviously Georgianna had been talked to by Darcy and she was anxious to meet Lizzie, maybe she made comment to her Lady. It would be assumed Lady Catherine kept well informed of the goings on at Pemberly. But again I still suspect Mr. Collins as the most likely of suspects!