Many decal sheets have all the writing and designs made with the decal film over them, even the company logo and writing identifying the particular sheet. These little images are not used on the model. If a kit is old and I suspect possible decal problems, I will test the sheet by trying to apply one of those extra decals to a piece of scrap. That way I will know if I need to apply decal film before I lose any of the decals necessary for the model.
I also now always scan decal sheets, so in the worst case, If I screw up any decals I might be able to make inkjet replacements.