Here’s why you have to actually proofread (and don’t assume spell check is good enough).
Whoever wrote out the results of an 1870 (or 1890, depending on where you get your information) German study misplaced a decimal point one spot to the right of where it was supposed to be and gave spinach ten times the iron than it actually had. It was written out by hand, so we can almost forgive the guy.
The bad part? It wasn’t until 1937 that some other German dudes discovered the typo and fixed it. But even then, it wasn’t really pointed out to the public until the British Medical Journal republished the study in 1981. Too late. Popeye debuted as a supporting character in Thimble Theatre in 1929, and has been hyping cans of spinach since. And actually, spinach eating jumped 33% between 1931 and 1936 in the U.S. which supposedly saved the spinach industry.
So all the kids who grew up being forced to eat their spinach can thank a cartoon and a typo.