There was the episode where Del became one of the Inbetweeners, Rodney was his own dad and nothing funny happened.
In the early episode where they build a bomb shelter, Grandad mentions his older brother (George I think). Never mentioned again by Grandad or Uncle Albert. But I think these little references are allowable in the early stages of a show. I doubt John Sullivan thought it would even make it to series 2.
There was also Damian who went from a sweet little kid to prick.