There are a couple of ways you can repair a cast if you choose not to throw it out. And probably a couple more I know nothing about.
The first...Sometimes when pouring a sprue mold, you may accidentally over pour flooding the spout. This can produce an air bubble which may fool you into thinking the mold is full. Then when you open the mold, you find half a cast. These are fun for customizing. Now if you are lucky enough to be able to achieve a full seal by reinserting the cast back into the mold, then first simply cut out the part of the cast that failed put it back into the mold making sure it is 100% seated, and fill the rest of the mold. You can have fun by changing the color...or not. I've had success blending the same color together seamlessly in 2 or 3 pours.
If its a Smush mold, you can do the same as the above. Cut out the bad, reinsert cast into the mold, and add more resin. Sometimes that's not an option either. Sometimes where the cast fail happens, particularly in sprue molds, prevents simply cutting out the cast fail and adding more resin.
If its a high detail area, depending on the amount of resin used for the complete cast, I might suggest simply scrapping the piece and checking the mold for what could be causing the problem and try again. Sometimes its not worth it to save and fix. How much time are you willing to invest to save the failed cast? Is it worth it? Its not always.
If its in an area that can be worked back into form with a little sanding and ingenuity, then I would suggest taking some clay and building a little bowl around the hole which you would then fill with resin. The clay bowl keeps the resin from running all over the cast and ensures the failed area is fully filled. The bowl doesn't need to be very high. Simply tight to the cast and slightly higher than the cast surface. You can easily remove the clay after the resin has cured. What doesn't easily sands off. You then sand/rework the detail back. This method works well for flat, geometrical surfaces or areas where detail is easy to rework manually.
Those are the methods I use when I repair casts. Of course, you don't need to have a cast fail to have fun with the cast. You could intentionally under pour or pour discriminately to achieve results of a more specific nature.