A cookie-baking recipe book (this one, specifically) has the following instruction in it, which is incorrect in a single spot:
If you have a fan-assisted oven, you will need to reduce the recipe temperature by 68°F. Fan-assisted ovens all perform differently, so always refer to the manufacturer's instructions.
Can you figure out what's wrong with the instruction just by looking at it, and what correction you need to make? The error is not a simple grammatical error, but it does have to do with language.
I first found this problem here, so I cannot take credit for it. However, the mistake in the sentence does exist independently in a recipe book that I've also cited.