What's something that people keep getting wrong? (Especially something they don't even know they're getting wrong)
It can be something as simple as wanting to enforce "mischievous" over "mischievious," or something much bigger like "the earth isn't flat". It can be funny or serious.
Also, even though I said "common", I'm definitely okay with domain-specific or more narrowly-focused ones, so if there's something that really bothers you about your particular job, discipline, or hobby, feel free to share!