The avaricious Mr. Greengrass had been using Forgetfulness Potions on his business associates for some time in order to make them forget he hadn't paid them. It was during one such business deal that he came up with his clever idea: he would modify the recipe to create a Forgetability Potion, which he would then administer to himself and his family. In theory, it would make the Greengrass family only seem relevant when they were physically present; otherwise, everyone would basically forget they existed.
Unfortunately, overestimating one's talent in Potions has been a common folly among Slytherins since Salazar's time, because the traditionally Slytherin craft has always been taught with significant House bias in terms of marks. Greengrass committed this error when he completed his Forgetability Potion - he made it much too strong. According to Astoria, her father would wake up most mornings unable to recall his own name, and nobody else could remember it long enough to tell him.
Astoria had taken a smaller dose of the potion, and this was a constant source of annoyance for Draco when they were an item. Some of Draco's acquaintances had been convinced that he'd made her up, even after meeting her several times.
- friend number three by riptey