I vaguely remember the "Baby" episode of Beasts, which someone describes really above. But it is the opening "Special Offer" episode that stuck in my mind more, mostly because the setting was so mundane.

Like most throwaway 70s telly, I only saw it once, but I vividly remember Pauline Quirke playing a checkout girl in a supermarket who became obsessed with the store's cartoon mascot (a rabbit, I think) called Brightway Billy. She started hallucinating about seeing the rabbit in the shop. I was 12 when it was shown and it totally creeped me out, and created an atmosphere of insanity and hysteria in a very bland setting. At school the next day, every was imitating Pauline Quirke's moon face calling out for "Billy!".