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.
Spinach extracts as dietary supplements
Around 1910, several dietary supplements based on spinach seem to have been widespread in Germany. At least two publications in the Pharmazeutische Zeitung suggest this:
1. Serger, H. (1906) "Über den Eisengehalt des Spinats." Pharm. Ztg. 51 (No. 33): 372.
2. Kobert, E.R. (1914) "Die Spinate als Arzneinahrungsmittel." Pharm. Ztg. 59 (No. 57): 422-423.
This was a reprint of the original:Kobert, E.R. (1914) "Die Spinate als Arzneinahrungsmittel." Beiträge zur Klinik der Tuberkulose und spezifischen Tuberkulose-Forschung (1914) Vol. 31 (No. 3): 481-489.
The latter journal metamorphosed into Pneumology first and finally into Lung: an international journal of lungs, airways and breathing.
Serger (1906) starts by saying that spinach (Spinacia oleracea) probably has the highest iron content among vegetables and is therefore recommended by physicians to anemic patients. He tested two dietary supplements for their iron content. These supplements were called Spinolum siccum and Extr. Ramkulini. In order to find out how well the iron was extracted into these products (and not discarded with the waste) he also tested fresh winter spinach and found an average iron content of 0.104% of the dry matter. That was three times the 0.0327% found by Bunge (1892, p. 181, see also here).
Kobert (1914), on the contrary, already claimed that spinach was not particularly rich in iron and that the leaves of kohlrabi were much better as an iron source. He had no particular beef with iron anyway, but suggested that the saponins in spinach were good for the lungs and for patients with respiratory problems instead.
Higher absolute iron content, but lower comparative values
This seems to chime with the earlier finding that Haensel (1909. "Über den Eisen- und Phosphorgehalt unserer Vegetabilien." Biochem. Zeitschr. 16: 9-19) denied that spinach had the highest iron content among vegetables based on own analyses (see here). That is, the average iron oxide content of the dry matter found by Haensel was 0.445%, whereas other vegetables had higher values, for example lettuce (Haensel 1909, p. 12, "Kopfsalat:" 0.679% on average).
The odd thing is that for spinach alone, Haensel's values are much higher than Serger's, even if we subtract the oxygen from the iron oxide. The portion of iron in iron oxide is 69.94%, hence Haensel's value of 0.445% iron oxide is equal to 0.311% pure iron in the dry matter. Why then did Haensel refute the idea that spinach was the richest in iron?
Haensel's conclusion was not about the absolute value of the iron content of spinach, but about its relative standing in comparison with other vegetables. It was aimed against the "vielfach herrschende Ansicht" (widespread opinion) that spinach was the richest in iron, and he could compare his value for spinach directly with his values for other vegetables. Serger had no direct comparison with other vegetables and probably just reiterated the widespread opinion. He also found that his finding chimed with that of "Königs Nahrungsmittelchemie."
Curiously, Haensel's value (equal to 0.311% Fe in the dry matter) was about ten times higher than Bunge's 0.0327% (see also here).
The claim that a particular value of the iron content of spinach is too high or too low is different from the claim that spinach has or has not the highest iron content among all vegetable. The first is about absolute numbers the second depends on context. Haensel's absolute value seem to have been roughly 10 times too high for spinach in comparison with other measurements, but the unexceptional place of spinach within Haensel's data set was correct.