it's more likely that the stat for equipping is calculated differently to when unequipping (ala, "same" code in two different places, and one got modified at some point and forgot about the other).

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

How would that be one of the easiest mistakes to make? There are no operations for working with percentages, nor is it a thing you'd want to do.

To make that helm work as you describe, you'd need to multiply the stat by 1.1 when equipping the helm and then by 0.9 when deequipping it. But at that point you've lost any justification for making the mistake; those are just two random numbers that don't cancel each other out. It's not like you're adding a number in one place and then erroneously subtracting that same number somewhere else.

(Also, these are very strange multipliers to use this way, because they can never produce exact results. You're bound to see rounding errors if you insist on update-when-equipping and update-when-deequipping instead of recalculate-stats-when-equipment-changes.)