One of the first things people who need to avoid gluten need to do is identify all of the sources of gluten in their diet. Unfortunately, there are quite a few hiding spots. The list below is by no means exhaustive and is only a suggestion of where to look for gluten in your diet. I will update it when ever I find other suggestions.
Flour - I still can't believe how many people don't make the connection that flour is made from wheat.
Mashed Potatoes - if a pot of mashed potatoes comes out runny it is not uncommon for a chef to add flour to them to firm them up.
Scrambled Eggs - some restaurants add pancake batter to their scrambled eggs to make them fluffier.
Fish - some restaurants dust their fish with flour to keep it from sticking (in the over, on the grill, anywhere really.)
Vegetables - some restaurants (especially Italian restaurants) steam their vegetables in the same water they use to prepare/reheat their pasta.
French Fries - some are coated with flour others are contaminated when they are fried in a shared frier. Read my full article on where and how to get Gluten Free French Fries.
Soups - it is not uncommon for the soup base to contain flour.
Meatballs - a lot of people put bread crumbs in meatballs (and meatloaf, and even sometimes in hamburgers).
Oriental sauces - both soy sauce and teriyaki sauce usually contain fermented wheat.
Crab - real crab is gluten free, imitation crab often is not.
Couscous - its a type of pasta.
Broth - some brands of broth contain gluten and some restaurants may use broth for rice, soups, steaming vegetables, etc.
At the supermarket:
Any non-whole food - really anything that is processed could conceivably contain gluten so always double check the ingredients list. Oh, and manufacturers do sometimes change their recipes so you have to check every time.
A flour by any other name . . . - sometimes only the strain of wheat will be listed, not the fact that it is indeed wheat - this includes einkorn, spelt, kamut, triticale, durum, semolina.
Non-wheat bread (for example: corn bread, potato bread) - these breads are almost always wheat flour based.
Malt, Malt Flavoring - both are made from barley.
Candy - some are made from wheat. Twizzlers for example.
Gluten free flours - According to Gluten-Free Living Magazine, if these are not specifically labeled gluten free they may be cross contaminated: Soy, sorghum, buckwheat and millet.
Vegetarian 'Meat' - a lot of vegan/vegetarian imitation 'meat' products are made from or with wheat.
Toaster - if your toaster is used/has been used with gluten containing products the crumbs could get on your bread.
Waffle Iron - It is quite hard to clean all those little nooks.
Condiments - if you share these with others crumbs could get mixed in here as well.
Wooden spoons/kitchen items - wood is porous, you should not use any wooden kitchen items when preparing gluten free food.
Makeup - quite a few contain wheat. (I know someone with celiac who had to have his girlfriend switch to gluten free lipstick.)
Medicines - You'll need to check with your pharmacist on this.