For many CFers, they can get hypoglycemic but their blood sugar drops to a certain point and remains stable, not dropping below any dangerous level. But for me, somehow my blood sugar can drop extremely low and so I'm at risk for a reaction because there's no glucagon back up system to save me. Studies have shown that the severity of delayed insulin response and hypoglycemic reactions does correspond to what kind of mutation you have . It is likely that just like delayed insulin production in CF, glucagon production is also delayed in reaction to hypoglycemia. So if I feel hypoglycemia coming on, I've got to eat some glucose or it won't be pretty. If you've got IGT or CFRD, it's a good idea to keep a small hit of glucose with you at all times, preferably a fruit like dried apricots or a fruit leather. I keep a fruit leather in my backpack and every one of my jackets to use in emergencies (and ONLY emergencies, they're too sugary for us under normal circumstances). If you've eat a moderately carby meal with a lower glycemic index and get a reaction, you're likely not at risk for passing out because your insulin overshoot won't be too big, since your pancreas won't be wanting to excrete as much insulin in reaction. But if you've just eaten a high carb meal with a high glycemic index, watch out and keep a snack with you for a few hours.
The take away message here is that reactive hypoglycemia is completely avoidable. What causes an insulin overshoot? Only higher carb meals with a higher glycemic index, meaning foods with quickly available sugars that hit the blood stream and cause a dramatic spike in blood glucose. This means sodas, candy, pastries, anything with a lot of sugar, any simple starches like potatoes, and anything with refined carbs (e.g. white flour) that are not eaten with a significant amount of fiber, fat, or protein to slow the absorption of glucose in the small intestine. I have a hypoglycemic reaction anytime I eat more than 30 grams of carbs in the morning, so I try to avoid eating carbs before noon. If we avoid these higher carb "foods" (most of these are toxins and should be avoided anyway) and make sure we choose to eat low glycemic index foods and meals, including fiber, fat, OR protein (but not all together), then hypoglycemia can be avoided. In addition, I believe that having a low-carb breakfast is very important in hypo prevention. Since the digestive tract is relatively clear after about 12 hours of fasting during the night, your body can absorb carbs a lot more quickly in the morning than at any other point during the day. This may be why I have historically only developed reactive hypo after breakfast. In the morning, the insulin response is not as quick as carb absorption, so there is greater risk of reactive hypo. Instead of loading up on cereal, oatmeal, a muffin, or a fruit smoothie in the morning, try eating a lot of fat or protein. A few eggs (if tolerated) plus bacon is a good choice, or some nuts or nut butter lathered on something fibrous like celery. My digestion is quicker in the morning, so a couple hours later I am ready for second breakfast, which can be slightly more liberal with the carbs.
The concept of the glycemic index is very helpful to use in conjunction with other dietary concepts that I discuss, and I suggest browsing through the food database to check our the glycemic load of the foods that you eat here.
By the way, cinnamon modulates insulin secretion and boosts insulin sensitivity, so when consuming something with a bunch of carbs or sugars in them, make sure to take cinnamon in pill form or add a lot of cinnamon to the meal. Cinnamon is truly effective, it may not prevent a hypoglycemic reaction with a high carb meal (more than 30 g of carbs). In addition, using digestive bitters before your meal is helpful to prepare the digestive system for your impending meal, increasing insulin sensitivity, and slowing upper-GI motility to reduce rapid absorption of carbs, which cause blood sugar spikes and crashes.
Also, here's a useful link that outlines a few of the long-term effects of having frequent bouts of hypoglycemia. I have noticed several of these symptoms myself. The most concerning of which is that I now don't feel the symptoms of hypoglycemia until I'm close to 40-50 mg/dL, whereas I used to get shaky and hungry at 60-70. This is not good, and shows that I am allowing myself to get too low too often, so I need to change my dietary habits, or else one day I won't be able to detect my hypoglycemia until it's too late and I pass out.
 Identification of insulin secretory defects and insulin resistance during oral glucose tolerance test in a cohort of cystic fibrosis patients. <http://www.eje-online.org/content/165/1/69.full.pdf>
 Insulin secretion, glycosylated haemoglobin and islet cell antibodies in cystic fibrosis children and adolescents with different degrees of glucose tolerance. <http://www.ncbi.nlm.nih.gov/pubmed/1761281>
 Cystic Fibrosis-Related Diabetes and Abnormal Glucose Tolerance: Overview and Medical Nutrition Therapy. <http://spectrum.diabetesjournals.org/content/15/2/124.full>
 Spontaneous hypoglycemia in patients with cystic fibrosis. <http://eje-online.org/content/156/3/369.long>