That's happened to me a couple times. The first time was my second shot but this last time was just a couple months ago. As far as I could tell, it really didn't cause any issues aside from me being irritated as hell that it happened in the first place. You'd have to consistently be getting half your current dose for a somewhat extended period of time to really experiences changes (in the wrong direction) or a lack thereof.
Anyway, don't sweat it. It happens and it will more than likely not be the last time it occurs.