I wonder if it was percentage based. That'd be one of the easiest mistakes to make.

- Player has a stat of 100

- Wears "+10%" helm

- Player has a stat of 110

- Removes helm, game subtracts 10%

- Player has a stat of 99

Actually, now that I think about it more, that wouldn't work as described above because once your stat got close to zero you'd enter a kind of Zeno's Paradox situation.