This is a literal shot in the dark because ive never used this script but im thinking about buying it because it looks great.
In the pictures i see posted, It looks like, if you count the 3 weapon slots (weapon, shield, and spec wep) plus all potions, and the rock cake, they sum up to the suggested number of inventory slots used. It looks like less of a script error but rather a user error. Albeit it could be resolved by enhancing the script. The script likely anticipates the user to not fill the inventory all the way, so that it has room for weapon switches when it needs to spec.