In the year 1870, a German chemist named Erich von Wolf made a typo in his scientific paper on chemical composition of spinach and placed the decimal point one-place-too-many to the right. This way he amplified the content of iron 10 times – from 3.5 mg to 35 mg (in 100 g of fresh spinach).
It took 67 years to recognize the error and even though it was corrected in 1937, the myth about spinach being an exquisite source of iron remained, and it was even brought to the big screen through Popeye, a fun sailor obsessed with this leafy vegetable. Even today, some doctors and nutritionists make (unfounded) recommendations to anemic patients on how to enhance their spinach intake as a way of amplifying hemoglobin levels in the blood.
Not only is the level of iron in spinach not that high, but it's also not very usable. Iron in plant-based food items is present in non-hem form, which is absorbed in a much lesser way than iron in hem form which can be found in animal-based food items.
I must emphasize that this does not mean spinach is a nutritive worthless food item. On the contrary, it's a good source of fiber, vitamins, minerals, and phytonutrients.