1) Loading up on soy milk when they give up dairy. The soy molecule is very similar to the gluten molecule which is similar to the casein molecule.) To see results, often a person needs to eliminate soy when they are eliminating gluten or dairy. Once the immune system and gut heals, then you can add limited quantities of soy back in, beginning with soy lecithin which has only traces of the protein. But loading up on soy at any point in the GFCF diet is not a good idea. See my post on Genetically Modified Soy or read The Whole Soy Story for more info on the dangers of soy.
2) Not taking a multivitamin. Most mainstream products are fortified with essential vitamins that people need to be healthy. If you switch from mainstream packaged food and fortified flour, you need to take a multivitamin to make up for the nutrients you will not be getting from the fortified food. EVERYONE on a restricted diet (whether GFCF, dairy free, gluten free, vegetarian or vegan) needs to take a multivitamin. There are products which contain mega vitamins for people who are not absorbing nutrients well from companies like Kirkman or just a mainstream vitamin from a company like Hero or Rainbow Light is a good thing to include in every child's diet.
3) Taking vitamins or medicine that contain gluten. Check the label or call the manufacturer or pharmacy to check if vitamins or prescription medication contains gluten. Visit GlutenFreeDrugs.com for more information.
4) Eating oatmeal. Oats technically don't contain gluten, but because of crop rotation they grow the oats on the same fields where wheat and rye are grown so there is cross contamination unless the oats are certified gluten free--which means they are tested for gluten. And some people don't do well at all with oats because of the phytic acid which binds to minerals like calcium. See my post on Whole Grains and Calcium for more info.
5) Eating french fries or fried food in restaurants. The fries might be gluten free, but is everything that is being fried in the oil also gluten free.
6) Loading up on Rice Dream or a product labeled "gluten free" that actually contains gluten in the processing. See my post on RICE DREAMS IS NOT GLUTEN FREE or my post on Gluten Free Labelling and the 20 ppm Standard.
7) They go off the gluten free diet because testing says they aren't allergic to wheat or don't have celiac. There are many many tests for gluten intolerance and celiac. Many tests aren't accurate--especially for kids. I know personally of five people who have had negative tests who ended up having celiac. Get tested before you begin a gluten free diet or do a gene test if you are already on a gluten free diet. But realize these tests can be inaccurate. The best test is how you feel.
8) They load up on gluten or do a gluten challenge in order to get testing done after improvement on the diet to be tested.My daughter and I made this mistake. We never did a gluten challenge with my son--he was the one who bounced back the quickest. The newer thinking on this, is that damage from gluten can be irreversible. It's better to just get better and do the gene test if you want more information. Ron Hoggan has an article Challenging the Gluten Challenge that explains why a gluten challenge is not a good idea.
9) Eating out. If you've ever spent five minutes explaining how you can't eat wheat/gluten and had a salad arrive with croutons or bread on top, you understand why eating out can be problematic. Ordering simple food that doesn't naturally contain gluten or going to a restaurant that is gluten friendly or has a gluten free menu is your best bet. I have also had the most success getting gluten free food by asking to speak to the chef personally.
10) They cheat. The gluten free/casein free diet must be 100%. In order to heal, you need months on the diet. In the words of Dr. Nancy O'Hara "If you are not going to do it 100%, I would rather you be on a different diet." It just doesn't work if it is done incorrectly.
My DH added an eleventh mistake people make...CROSS CONTAMINATION. If the kitchen or restaurant is full of gluten then you might inadvertently be getting traces of gluten if things are not prepared carefully. If everyone in the household is using the same jar of peanut butter and some people are spreading it on bread and some people are spreading it on gluten free bread, there is a chance of the gf person getting gluten.