"Enormity" is one of those words. It used to be strictly "horrible," conveying a sense of wickedness, and to use it as a synonym for "enormousness" was glaringly wrong. But the mistake was so common, even among journalists and writers, that that it has gained acceptance in at least one English dictionary.
What I mostly get riled up about are words or phrases that are just plain wrong, but that have been used so widely that some dictionaries have accepted them as correct.
Eg: https://www.youtube.com/watch?v=om7O0MFkmpw