One reason I never ever throw away a scrap. Once I needed two pieces and made some "new fabric" out of the scraps. I cannot even see the seams in the top.
In one kit from a garage sale, I was missing one block from a BOM. I made the rest and figured I would have to do scrappy applique at the end. I had an assortment of fabric left so I contacted the Fat Quarter Shop and they sent me the pattern for the old BOM. I used other fabric from the kit and stretched it.
If a border was short, I might try an asymmetrical addition or even find a solid that would work.
NOTE: I never buy full price kits but only on ebay or garage sales/estate sales. So I have no recourse but don't pay much and figure the fabric is what I am buying and the kit feature is extra.