Often ingredients in processed foods can contain meat, fish or egg products. Many people define vegetarian dishes to include just no meat or fish, while eggs are considered to be vegetarian.

Therefore if you ask if a product that contains egg is vegetarian, you may be told that it is vegetarian.
Often, even though a dish may appear to be vegetarian, it may contain ingredients that are not vegetarian. When a food contains natural favors, it may be derived from meat. Different chemicals and additives such as mono- and diglycerides may be derived from either meat or vegetarian sources. Cheeses may be made with an enzyme that is derived from either meat (cow's stomach) or a vegetarian source.

Vegetarian foods may be cooked on a surface that may also be used to cook meat. So if you are interested in maintaining strict vegetarian principles, it is important to ask what surface grilled dishes such as veggie burger are cooked on, and whether the grilled dish is cooked on the same surface as meat. Often meat is cooked on the same grill as what a veggie burger or grilled cheese are cooked on.
Also foods that are cooked in oil may have meat cooked in the same oil. So it is a good idea to ask when ordering French fries or other such dishes, whether meat, chicken or some other non-vegetarian item has been cooked in the same oil.