I have previous with casino’s and sanding, been there and it takes time to get it right. The only thing you can do when that happens is to paint it with a solid color.
The spots you sanded through will suddenly show up as dark splotches. But if you keep sanding, you’ll almost certainly sand through in spots (especially on curved areas on arch tops) and not even notice it until you apply the stain. If ANY poly remains it will not take stain, and you’ll get a splotchy look. Trust me, I learned the hard way that it’s nearly impossible to sand off the finish to the point where the guitar can be stained without going through the Maple layer in spots because the clear poly actually has soaked into the Maple layer, essentially making the stainable layer paper thin.
