I activated an "Offense and Defense +50% (7 days)" item, but the effect disappered. What happened?

This can happen if you try to activate two incompatible items at the same time.

For example, if you activate an "Offense and Defense +50% (7 days)" item followed by an "Offense Enhancer +10% (4h.)" item, the two items are incompatible because they are both attempting to enhance the same thing. Since the "Offense Enhancer +10% (4h.)" item was activated second, that effect overrides the "Offense and Defense +50% (7 days)" item.

A warning is displayed whenever you are trying to activate an item that will override an existing effect. It looks like this: