This is a very interesting question! I assume you are just talking about nail polish?
From a health and safety point of view, there is nothing intrinsically wrong with tipping the end of one bottle into another. No pathogens can survive in nail polish.
However, ‘use by’ times on such products are there because the quality and performance can possibly be affected over time. For nail polish, it is often 24 months after opening. There is likely to be a lot of evaporation so the polish will be thicker, maybe even unworkable. And yes, it could be taking some dust and glitter with it.
My suggestion to those wanting to use the end of a favourite colour would be to add a few drops of the base coat from the same range. No solvents or polish thinners. After a good shake, the thick polish will be more workable. It will probably lose some of its gloss but this is rectified by using a top coat.
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