An insulin reaction is an emergency, and it needs prompt attention and the immediate administration of glucose. Administer karo syrup, icing, pancake syrup, etc some quick form of glucose immediately.
If the pet cannot swallow; administer by rubbing it on the gums in the mouth and it will be absorbed.

What causes hypoglycemia?
too much medication (error in dosage)
a missed meal 
a delayed meal 
vomiting of the meal
too little food eaten as compared to the amount of insulin given
strenuous exercise(chasing a squirrel around the yard for an hour)
taking certain medications (bactrim, etc)
stress (though this can also raise glucose)

What are the symptoms of hypoglycemia?
shakiness 
dizziness 
sweating (panting)
hunger 
trembling lip (Queenie's bottom lip quivers)
stumbling, staggering,  clumsy or jerky body movements 
difficulty paying attention, or confusion 
excessive sleepiness and lethargy

What do I do now that my furchild had a hypoglycemia attack?
You need to contact your veterinarian immediately for adjustment in the insulin doseage.
A full blood glucose curve should be done to determine how low the level of glucose is falling to in the bloodstream.

Some owner's firshand accounts of hypoglycemia in their pets!
From Bonita and Ljuba (diabetic dog)
Regarding hypo's - last week one night, Ljuba didn't come to greet me when I came home from an exercise class ... she just stayed on her pillow on the floor in the family room.
In response to whether she had been out at all in the hour I had been gone, hubby replied she hadn't moved. I carried her outside, put her down and she wouldn't move!!! Now I knew for sure something was wrong.  A quick blood test revealed a 32 (!!) Thankfully, I know my gadget is 20-22 lower than my vets and I send a "thank you" kiss to the person who gave me this little edge.
In retrospect, I realize that Ljuba probably didn't want to move for fear of falling on her face.
I offered her some of her food, which she quickly scarfed down - and then some more til she had eaten aprox 1/4 to 1/2 of a regular meal.  Then I kept a close eye on her. In 30 minutes she was up to 80 and in another 30 minutes, she was at 110.
I probably overfed (over-reacted), cause she was high in the morning.
Although this time her numbers were very low, this scenario, unfortunately, is quite common in our household in the evening. Meaning that Ljuba uses her insulin in the evening differently than the morning.  Go figure - same dog, same food, same insulin (although a reduced dose).
Ljuba also sends a signal when she feels that drop - she runs into the family room and sits next to Greg's recliner for a moment ... making eye contact with me, if  I'm on the couch. Then she runs to the water dish we keep in the family room and gets a big drink. This is my signal to do a quick test and offer a snack.
From Rhonda and Missy (diabetic dog)
Missy had a hypo at 3 am in the morning. I just had checked her all seemed ok went to lay back down and I heard this noise, I flicked on the light and she was in a seizure. I ran for the Karo and gave a little on the gums a little at a time within seconds she was ok. I then gave her a little food. I'm home all the time as I do not work, Missy runs low usually never high. I watch her closely, when Missy gets to low when she stands her back end sorta sways a little like she is off balance I then give her a cookie and that seems to do the trick. I wouldn't use Karo inless she was really wobbly and had a hard time standing or if some food didnt help because I'm home here to watch. Missy can run into the 40's with no signs of hypo. I do not home test only urine so she goes for her curves every 6 weeks. I don't think there is a magic number when to give Karo because each pet reacts different. Missy is ok at 40 where another pet at 40 could be in a seizure. Our own judgment is our best guide because we know our pets and were so in tune to them. 
From Edward Murray and Anna (diabetic dog)
I also look at the clinical signs, rather than the BG numbers.
Each pet will act differently, but typically, the way things work when they are hypo is that they first are in a desperate need of food. They know they are going low. Then if they stay low long enough, they will either start staggering, or more often, they will start staring blankly. They lose their vision.
As long as they are not in a seizure, it seems a good rule is to try to feed them something like a dog biscuit or their regular food and watch them carefully. Food takes almost an hour to be digested so the effect is not going to be immediate, but if you have caught it soon enough, this should be okay.
However, if it obvious that things are getting worse, they fall down or go into a seizure, then it is time for the Karo. By rubbing it on their gums, it is absorbed immediately by the body. I know from a recent experience that at least not all cats will respond to Karo. For some, it seems that maple syrup or honey is needed. I have not heard of this being true in dogs. And of course, you are the second person I have heard of using icing!
Again, watching the clinical signs of what is happening, I would try to give as little Karo as is necessary to keep them from going lower. I do this with Anna in small doses by sticking my finger in the Karo and rubbing that on her gums until it is clear that she is coming up.
As soon as she is coming up, I stop the Karo and feed.
Karo will send the BG's shooting up, but the effect wears off quickly so you want to get food in them as soon as they are willing to eat and not showing signs of deteriorating. As Rhonda said, we are each probably in the best position to judge this because we know our guys probably better than anyone
else. And the BG's are just a guide.
At the low end of the scale, the meters are set to read low by about 20 points to try to prevent accidental hypo incidents. Further, the capillary blood we get for BG's will also read low by about the same amount. So when you see a reading of 50, this in fact may be a true BG of 90. They won't
start going hypo until they are under 60 in most cases.
The specific answers are that Karo acts almost instantly, food takes about an hour to show up in the BGs and it is the clinical signs, not the BG that are important in managing a hypo incident. Food is whole lot better than Karo if they are not in a seizure or clearly deteriorating. The amount of Karo is as little as you can give them and still reverse the symptoms.
 
from Lee
Date: Fri, 21 Jul 2006 
I am writing this to let you know that this can happen to any one at any time.  I don't know if I miss measured the insulin this morning, the heat, the food mix,  or maybe everything.  But 1.5 hours post shot I  noticed she had started hypoing.  Of course the Karo was missing in action, so I had to use sugar.  Poor thing had her gums sanded with sugar and water.  I don't know how low she got as I didn't get the meter out until I was sure she was recovering, but her BG started coming up and I got on with my day.  
 
This is the first time in over two years (dx 2/04) this has happened.  I have always been happy that she was easy to manage and I hope this isn't the start of some rough times, but we will handle it if it is.  

Note from Judy
Go get your bottle of karo syrup(corn syrup) and try opening the lid right now to make sure that it is not stuck because you never know when your pet's blood glucose might drop to low!!!!