When talking to Dr. Becker over the phone, Del says that the doctor doesn't know him. Yet, in the very next scene, Del mentions it was Dr. Becker who arranged for the Trotters to move to Nelson Mandela House, and in 'Thicker than Water' (1983), Del explains to Rodney that Dr. Becker was the family doctor, so he must have met him before.
Grandad says his grandfather carried the cigarette case throughout the Boer War which lasted from 1899-1902 but then says he died at Rorke's Drift, fighting the Zulus, which was fought twenty years earlier, in 1879.
When Grandad is in bed, he tells Del and Rodney that he has never had a garden. But in "The Russians Are Coming (1981)", Grandad's allotment was mentioned, and in "Mother Nature's Son (1992)", the Trotters actually go to Grandad's allotment, which would surely count as a garden.
The bed Granddad sleeps in is a single. However, in "If They Could See Us Now.....! (2001)", Del explains to a horrified Rodney and Cassandra that Grandad had his first fit in that bed, but the bed in that special was a double.