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 endeavor was full of data handling errors, though none as simple as a misplaced decimal point.
The 5th, corrected edition of Joseph König (1920. "Chemie der menschlichen Nahrungs- und Genussmittel... Berlin. J. Springer.) swapped iron contents in dry with those in fresh matter at page 453 as follows:
"Der Spinat wird vielfach als eisenreiches Gemüse angesehen und empfohlen. [Irrelevant sentence omitted.] In der Regel enthält aber der Spinat nicht mehr Eisen als andere Gemüse, nämlich nach Haensels Untersuchungen in der Trochensubstanz 0,03%, während Winterkohl in der letzteren 0,056%, Kopfsalat 0,054% Eisen ergeben. [...]"
"Spinach is often regarded and recommended as rich in iron. [Irrelevant sentence omitted.] Usually however spinach contains no more iron than other vegetables, according to Haensel's investigations 0.03% in the dry matter, while winter cabbage contains 0.056% and lettuce 0.054% iron in the dry matter."These values, however, are averages of Haensel's (1909) values for fresh matter. Haensel's values for the dry spinach amounted to 0.445% iron on average. König combined the correct statement that spinach is not exceptionally rich in iron with a values that was 15 times too low according to the state of knowledge of that time.