This hasn't happened at the range yet, but when I'm practicing at home, I'm finding it harder and harder to focus on the front sight. It's not that I can't focus, exactly, but that the image from my non-dominant eye is getting more and more pronounced, so what I see is the front sight standing beside the rear. When I close the non-dominant eye, it turns out I have perfect sight alignment, so it's not throwing my aim off. And when i do the dominance tests, I keep landing on the same ol' dominant eye that I always have.
Lighting in my house isn't all that bright in the evenings when I practice, which may be the reason. But I am getting to that certain age when vision changes are often reported.