What happens when you lose an invention that has a one-shot benefit?
On page 16, of the Portal Edition rulebook it states that when flipping a item card over, that "If the Item had an additional effect, you should cancel it
if possible."

So if you lost, say, the Spear, you should reduce your weapon level by 3, if possible.

Earlier on the same page, it mentions the phrase "if possible", and clarifies that if you cannot pay that cost, nothing happens, and the Rule of Unfilled Demand does not apply.