Things That Annoy
1. "Correlation is not causation."
What this is meant to convey: Just because X and Y are correlated doesn't mean that X causes Y.
Example: Doctors have observed a positive correlation between shoe size and IQ score for children. Most people would reject the explanation that big feet cause increased intelligence, because a more reasonable explanation is that as children get older their intellects mature and at the same time they are growing physically and their feet are getting bigger. However, it's not correct to make the general statement that correlation is not causation, because, as this counter-example shows, correlation sometimes does suggest causation: Doctors have observed a positive correlation between the number of blows to the head and the severity of headache pain in the victim. Not many people would bother to look for another cause for this relationship: it's reasonable to conclude that X is correlated with Y, because X is causing Y.
Corrected version: "Correlation does not require causation."
2. "I could care less."
What this is meant to convey: I don't care at all.
Example: Ever since Thompson dropped out of the race, I could care less about the coming election. Logically, however, this implies the opposite of the intended meaning. If there is still capacity to care less, it means I care more than not at all. (Somebody actually created a chart to show how this works.)
Corrected version: "I couldn't care less."*
What are your grammatical pet peeves?
[* Prediction: Some reader will comment to the effect that he "could care less" about my analysis.]