When you know something to be wrong and you correct the person in private?
Spelling errors drive me nuts.
Valence for Valance. 2 different things entirely.
Vial for Vile. (edited because apparently I don't proof read very well)
Stalk for stock.
How freaking hard is it to open a dictionary either hardcover or online and look something up?
Washing soda and Baking soda. 2 different chemical compounds and turning one into the other doesn't quite do it. (I knew sometime in my life all those long days in Chemistry in 11th grade would pay off).
Stupid home recipes like coconut oil on dermatitis and ground chickpea flour for blemishes. Oh God.