This is somewhat dangerous and should be done only with certain precautions. In most people,it is only a small percentage of the total dose, so it doesn't make much difference. Some very unusual people, however, have a tremendous degree of subcutaneous insulin degradation. I've seen only one of these people in my life. You can inject insulin with an enzyme inhibitor called Trasylol, but it's expensive, and it's simpler and cheaper to give the extra insulin.
Human insulins are shorter acting than animal insulins ofthe same type.
Each day is metabolically different. This is a very catchy phrase to emphasize that there are many variants and to explain why some days BG's may go up or down without a good explanation. It's best to wait 3 days before making changes in insulin doses just to make sure that it's a permanent change and not just a fluke of one day. Insulin vials should be kept in the refrigerator until you start using them. In this way, they may be used until they are finished.