It is weird that it took me so long to understand the end of the vial insulin effect. As a diabetic a day of higher than average blood sugars is a surprise but not out of the ordinary. Much like everyday life diabetics have bad blood sugar days. Last year I started to notice that these bad blood sugar days would always happen when I was using the last bit of insulin in the vial. After speaking with other diabetics I learned that the end of the vial is not as fresh as a full vial. I'm sure people are shaking their heads but I never thought of that.
Obviously the more insulin sits unused the weaker it becomes, sunlight and heat also effect the strength of insulin. To help combat this problem I was told to inject the last quarter of a vial in to an unused vial, this way the older insulin is mixed in with fresh insulin and that will help eliminate the bad blood sugar day.
It is also very important to keep an eye on the insulin in your pump. I have found that if I go more than four days without changing my pump reservoir the insulin will not work as well as it should. Keep an eye on your blood sugars and if your numbers are out of the ordinary check your insulin!!!