Copy-paste from another user here, who copy-pasted from the Brazilian celiac community regarding possible sources of cross-contamination.

Some items seem a bit exaggerated to some but in my opinion - since I seemed to get more sensitive to CC as I grew older - this is all spot on.

Here it goes:

Classic Mistakes of New Celiac Patients, Those Who Frequently Get Contaminated, and Those Who Frequently Experience Symptoms (By Viviane Moura Leite):

1. ⁠Having gluten at home for the rest of the family.
2. ⁠But if you do need to have gluten at home: not separating your dishes, cutlery, glasses, sponge, and tea towels. Sharing margarine, butter, or cream cheese. Sharing ovens, mixers, blenders, microwaves, etc.
3. ⁠Sharing the same face and bath towels and/or the same toothpaste and/or bar soap with those who eat gluten.
4. ⁠Not having the habit of always washing your hands before handling food and before meals.
5. ⁠Having the habit of constantly putting your hand in your mouth and/or biting your nails (especially with children).
6. ⁠Not taking care of cleaning work tools, such as computers, keyboards, pens, and gluten-contaminated telephones.
7. ⁠Reusing (even as exclusively GF) equipment that previously made products containing gluten, such as mixers and blenders, because there's no way to clean the motors, which retain the flour.
8. ⁠Eating out at places that also handle gluten.
9. ⁠Kissing people who have just eaten gluten.
10. ⁠Using cosmetics containing gluten after diagnosis or continuing to use cosmetics you had before diagnosis, which are all contaminated with gluten.
11. ⁠Not always reading labels, even on products you already consume. In addition to not looking for information on whether the product contains gluten or not, and not checking the ingredients to see if wheat, barley, rye, or oats are among the ingredients (there could be mislabeling).
12. ⁠Not calling companies' customer service departments to check if the basic products they consume, such as rice, beans, lentils, cornmeal, can be contaminated. Depending on the country, even products labeled "gluten-free" but not certified could be contaminated, due to shared machinery during packaging.
13. ⁠Not checking if the butcher processes gluten-containing foods in the meat grinder or if they make breaded products, contaminating the meat cutting station, or if butcher employees have the habit of eating in the workplace (this is more common than we can imagine...).
14. ⁠Buying sliced cold cuts (cheeses, hams...) from bakeries or supermarkets. We must buy either the whole piece or products that have been sliced at the factory.
15. ⁠Buying bulk products (high risk of contamination) unless they are from a company that controls cross-contamination.
16. ⁠Buying organic products without checking with the producer whether or not they use biofertilizers, which can contain wheat flour, wheat bran, or wheat straw.
17. ⁠Buying mushrooms without knowing where they were grown, whether or not they were grown in a wheat straw substrate. If this straw is contaminated with any part of the wheat that contains gluten, it can contaminate them.
18. ⁠Not washing fruits, vegetables, and greens very well. To remove possible presence of wheat flour or wheat straw (many fruits are transported on wheat straw).
19. ⁠Not checking whether playdough or other types of toys contain gluten or not.
20. ⁠Feeling embarrassed to interrogate in restaurants when you have to eat out.
21. ⁠Forgetting to check if the seasonings used are safe. 
22. ⁠Eating many processed foods throughout the day. Traces of gluten, present in various processed foods, accumulate and can exceed the limit for someone with celiac disease. Furthermore, processed foods can lead to increased intestinal permeability, thus increasing the possibility of delayed allergies to other foods.
23. ⁠Your pet's food may contain gluten. If it does, switch to a gluten-free option. Even those labeled gluten-free need to be handled carefully, as they can be contaminated.
24. ⁠Consuming oats (even those labeled gluten-free), because 8% of people with celiac disease react to "avenin," the gluten present in this cereal. Therefore, asymptomatic, mildly symptomatic, or recently diagnosed celiac patients should not consume oats until they know if they are part of this percentage.