The old narrative
A misplaced decimal point caused the false reputation of spinach for being the vegetable that was richest in iron. Though still highly popular, this narrative is most likely wrong (see here). The decimal error probably never occurred in that stupidly simple way. Ignoring wrinkles in the narrative, such as that spinach is still rather rich in iron but that it cannot be assimilated well for other reasons, the new narrative can be stated most simply as follows.
The new narrative
The decimal error is a myth, it never occurred. The false reputation of spinach was due to unreliable methods or poor experimentation. That is, errors were inherent in experiments not data treatments (see here).
The complex history
Still, not everything about the spinach-iron legend is clear yet. In particular, nobody has yet thoroughly reconstructed where the original data came from, how they have been treated (mathematically) by the various researchers who wanted to reach comparability with their own data, and whether any mistakes were made in these data treatments. At the end of this series of reconstructing data handling, you will see that the whole research endeavour was full of data handling errors, though none as simple as a misplaced decimal point.
J. König (1904. "Chemie der menschlichen Nahrungs- und Genussmittel." Vol 2, p. 353) gives the range of iron contents in spinach as 32.7 to 39.1mg per 100g dry mass (see table below). The text above the table says that these values are from Bunge and Häusermann.
And indeed, Bunge found 0.0327g in 100g dry mass (see here), which is the same as 32.7mg in 100g. The other value of 39.1mg/100g is from Emil Häusermann (1897. "Die Assimilation des Eisens." Zeitschrift für Physiologische Chemie 23: 555-592). Like Bunge, Häusermann's main aim was to induce and study the anemia in animals, but he lists the iron contents of vegetables at the end of his publication. The value for spinach can be found at page 588 and it is taken, in turn, from Boussingault (1872. "Du fer contenu dans le sang et dans les alimants." Comptes Rendus de l'Académie des Sciences 74: 1353-9), or rather from Bunge (1892), who had taken Boussingault's data and transformed it according to König (1889, see footnote: **** here).
Later, J. König (1926 "Nahrung und Ernährung des Menschen. Kurzes Lehrbuch. Berlin: Julius Springer, p. 31) tried to combine these values with those of a later publication by Haensel (1909. "Über den Eisen- und Phosphorgehalt unserer Vegetabilien." Biochem. Zeitschrift 16: 9-19) it seems. Haensel compared his data for various vegetables and concluded correctly, that spinach was not the richest in iron, despite the fact that his particular value for iron in spinach was roughly ten times higher than Bunge's (see here). König seems to have grappled with this inconsistency in the research record as follows:
"Eine besondere Bedeutung wird auch dem Eisen in der Nahrung zur Bildung des Hamoglobins zugeschrieben. Von dem durchschnittlichen Eisengehalt des erwachsenen Körpers von 3g sollen etwa 1/6 auf Hamoglobin entfallen. Hiervon werden täglich 80-100 mg in Freiheit gesetzt. Diese werden von Leber, Milz und sonstigen Drüsen größtenteils gespeichert, so daß der tägliche Bedarf in der Nahrung nur 20-30mg betragen solI. Solche Mengen Eisen sind auch wohl in einer gemischten Nahrung vorhanden. Die größten Mengen Eisen finden sich in grünen Gemüsen, nämlich in 100g 30-60mg Fe203 (die Höchstmenge im Spinat); andere Nahrungsmittel enthalten nur den 10. Teil und noch weniger." König (1926, p. 31)
"Special significance for forming the hemoglobin is attributed to the iron in the diet. About 1/6 of the average 3g iron in an adult body if in the hemoglobin. Of these, 80-100mg are liberated daily. These are mostly stored in the liver, spleen and other glands so that the daily requirement in dietary iron amounts to 20-30 mg. Such amounts of iron are also well available in a mixed diet. The largest amounts of iron can be found in green vegetables, namely in 100g 30-60mg Fe203 (the maximum quantity in spinach); other foods contain only the 10th part of this and even fewer." (my translation)At page 82 König cites Haensel (1909) and another, but not on spinach. My translation below is not garbled, the original is:
"Unter den Mineralstoffen wird auch dem Eisen der grünen Gemüse wegen der blutbildenden Eigenschaft eine besondere Bedeutung zugeschrieben. R. Berg fand in 100g frischem Gemüse Spuren (Wirsing) bis 150 mg Eisen (Bleichsellerie); E. Haensel 3,9 mg (Zwiebel) bis 68,9 mg Eisen (Kohlrabiblätter) oder 31-37,9 mg Eisen in 100 g Trockensubstanz."
"Among the minerals a special importance is also attributed to iron of the green vegetable because of the blood-forming capacity. R. Berg found in 100g fresh vegetables tracks (Savoy) up to 150mg iron (celery); E. Haensel 3.9mg (onion) to 68.9mg iron (Kohlrabi leaves) or from 31 to 37.9mg of iron in 100 g of dry matter."
Coming full circle, page 86 correctly states that spinach contains no more iron than other green vegetables and refers back to page 82, where, as we have seen, nothing is said about spinach.