When restaining a body (in the same/similar color) which was finished in stain to begin with, you can run into the following problem:

You've block sanded the original body down to where the sheen of the top coat is gone and and applied the new stain evenly across the side your working on. As it dries you notice that small or even large areas didn't take the stain well and are developing light imperfections on the surface.

Don't panic, what has happened here is that the top coat was absorbed into the finish deeper than you thought it was.

First let the stain dry completely. Then take a piece of fine grit sandpaper and scuff down the surface even more where the spots are showing till your sure there is no more coating blocking the stains ability to absorb. Now take a cotton tip swab and lightly moisten it with your stain ( I know they suck up stain like a sponge). Wipe the tip of your swab against some scrap newspaper till it is almost dry, then gently rub the color into the spotted area. This way you can control the amount of stain that actually is absorbed into your project blending the imperfection(s) away. This sure beats sanding the hole thing down again and starting over!

Just as an added extra when staining figured tops:

Stain them twice (though most do anyway), but make the first coat very dark i.e. don't dilute it too much, then sand when dry to 400 grit removing much of the stain as you go along, then re-stain with the color diluted correctly to achieve your end color.
The point is that the first "dark" coat will bring out the figure of the wood much more after being sanded and the final more diluted coat applied.