Lycanthropy curse that comes with a kind of probabilistic/antimemetic effect that makes it especially difficult to remember when the next full moon is.
It doesn’t cause the one afflicted to forget that they are a werewolf. In fact, when they do transform, mentally they are still very much themself. It doesn’t even make them forget the time they spent as a wolf after changing back. It just makes it so that, through “bad luck”, things always come up that make keeping track of things like the phase of the moon much harder. Always some sort of stressful thing to worry about, or some big thing planned, that pushes thoughts of the curse out of the person’s mind.
The result is that the transformation is always a surprise. That trope of “the full moon was TONIGHT??” that I’ve seen in so many tf stories happens to the afflicted individual every single time, without fail. And since their mind isn’t altered, they get to fully experience whatever embarrassing situations arise from their untimely transformations.
Maybe if they took really extreme measures to keep track of when they’d transform, they’d be able to push past the curse’s power just once, and successfully not get surprised by it… but would it be worth it to go through all that effort every single month?
Weirdly enough, it seems like whatever it is that “just so happened” to keep them from expecting their transformation always goes unusually well. Whatever was stressing the afflicted out always resolves fairly nicely, or if it’s something they had planned, it always goes smoothly enough. In fact, a lot of the time, these things will resolve while they’re transformed…
Maybe it isn’t so much of a curse after all?