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 experimental methods not data handling (see here).
The complex history
Still, the new narrative is not the last word either. In particular, nobody has yet found that misplaced decimal point. Throughout this series of reconstructing the data handling of the primary research literature, I claimed that the whole research endeavour was full of data handling errors, but none of these errors was as simple as a misplaced decimal point.
The simple error
While the statement about the primary literature being a mess of poor data handling remains true (check it out) I stand corrected on the claim that none was as simple as a misplaced decimal point. A simple decimal-point error did occur in an article published in—wait for it—Science 90, no. 2347, pp. 596-597 (1939). But the research reported there was about the iron content of dried peas and beans instead of spinach.* So how can it be the mythical error recurring in the urban legend about spinach & iron?
[* The full reference is: Aschman, Leah, Mary Speirs & Dorothy Maddox (1939) The availability of iron in dried peas and beans. Science 90, 596-597.]
Let's first take a look at the error itself. Aschman et al. (1939) report the results of an experiment that was pretty similar to the much older experiments already done by Bunge (1892), for example. They rendered baby rats anemic by feeding them on milk only (mammalian milk is poor in iron). Then they fed the anemic rats dried peas and beans, did the proper controls and measured the recovery of the rats. Along the way, they also measured the iron content of the dried products and gave the values as follows:
Anybody with a sharp eye will spot the decimal-point error in the value given for butter beans at the end. And that is what the staff of the Nutrient Section of the Bureau of Home Economics did, when they published abstracts in the Journal of Home Economics 32, no. 7, p. 481:
This is probably the ultimate source of the urban legend that spinach got its false reputation for iron-richness through a misplaced decimal point. This particular source of a decimal point error could even explain why Bender (1977. The Spectator. 18 July, p. 18), the researcher who first started to spread that urban legend about spinach, iron and decimal points, associated the real but innocuous decimal point error of Aschman et al. (1939) with the name Schupan [sic] in his memory. Werner Schuphan has indeed published on the content of oxalic acid in spinach (see here).