Yesterday morning, I used a product in a tube that had separated so that reminded me that I should consider doing an entry about this. By a separated product, I mean that it's a cream and what comes out at first is an (often) oily clear (or near clear) goo and then the product itself.
Products are meant to be used regularly so, if you're not using regularly, it may separate. Depending on the product, the length of time before separation will happen can be very different from product to product. Normally, it may take a few weeks for it to happen but it could happen quicker than that.
If you haven't been using a product from a tube in a while, you may want to massage the tube first (with the cap still on). It sounds weird but it will help remix the product together. It's not going to make it all remixed but it will definitely help. Obviously, you will want to be gentle when massaging the tube as you don't want it to explode all over. That would defeat the purpose.
For all that you may feel that it's wasteful, you may need to throw away the portion of product that will come out that is separated. While you can certainly apply it and it shouldn't have any nefarious consequence, it just will not do its job properly. For example, separated eye shadow primer just won't hold onto the eye shadow as well as it should if the product is not the perfect formulation.
So the bottom line really is that, for all that you want to conserve products, they are meant to be used... or else you'll actually need to throw some away.