Since the early 1900s, Popeye the Sailor, who became particularly strong by eating spinach, has been held up to children as a role model. It has been suggested that consuming spinach meets our iron needs and thus contributes to good health. But if you look into the original literature, you discover that both were myths. Both Popeye, who is said to have become strong from the iron in spinach, and the high iron value of spinach itself. In the cartoon by EC Segar, Popeye himself says "Spinach is full of Vitamin A. An' tha's what make hoomans strong an' helty!".   

With the myth about the iron content of spinach, it's a bit more complicated, here one myth replaces another. The British Medical Journal published in 1981 that when the iron content of spinach was determined in the 1930s, the decimal point had accidentally slipped one place to the right. This decimal place myth is still present today in many publications on the subject of iron content in spinach. In fact, the excessively high iron content of spinach is due to the failure to take into account the difference between dried and fresh spinach.  

In the case of supposedly generally valid facts as well as assumptions, one should always critically examine the available literature and, if possible, refer to the original or primary sources.