Not quite a running gag, but worthy of note:
Three regulars from Roseanne became at least recurring characters on BBT.
Leslie Winkle was Darlene Connor, Rosanne’s daughter.
Leonard was Darlene’s boyfrield-then-husband, who lived with the Connors.
Ms. Cooper was Roseanne’s sister.
Howard Wolowitz considers himself a biker because he rides a Vespa.