One of the most enduring stories associated with the Renfrewshire witch hunt is that of the final curse laid upon the town and its people by Agnes Naismith and Katherine Campbell. Both women went to the scaffold allegedly in a great rage, and unrepentant, calling down the wrath of God on those who stood witness to the scene.

The laying of an iron horseshoe on the place that marked their burial location at Maxwellton Cross is a reflection of how seriously some people took the curse. Horseshoes are traditionally known as being symbolic of good luck, while iron has a long tradition of being seen to have positively magical properties. It is unlikely that the Church had anything to do with the laying of the horseshoe. This practice would likely have been viewed with suspicion by many local clergymen. Instead, it would have been laid by the ordinary folk in the community, many of whom would have taken a dying witches' curse very seriously.

It was said that if the horseshoe was ever removed, then Paisley would go into decline. At various points since 1697 the horseshoe was removed, and it was said that the town was beset with numerous accidents, mishaps, and even deaths. The original iron horseshoe no longer sits in its place protecting Paisley, having been lost decades ago. A replacement, although not made of iron, now sits in its place.