It's been a while since I found something to have a mini-rant about, but this is such a beauty it has to be featured. Behold the following key from "Funga Nordica" in the Marasmius genus:
If you had, for instance, a toadstool which was 17mm in diameter you would assume that you had gone down the wrong side of the key according to question 3, which has as a MAXIMUM a cap of 15mm across both sides of the dichotomy. If, however, you persevered and went to question 4 anyway, you would find a fruit body could have diameter 20mm. How?! These lines are right next to each other - how could nobody notice there's a glaring error here? If the fruit body can be 20mm wide in question 4 then it can be 20mm wide in 3b.
Fortunately my fb was 25mm in diameter, so I was in the wrong place one way or another - maybe.