Confusing sufficient with necessary is the LSAT's most common flaw, but this one is close on its heels. If you can master these two issues, you've probably nailed 1/3 of the questions on the LSAT's logical reasoning. This is the second concept that I teach to every class, and it's also very simple. I'll give you two examples of dumbass arguments that appear on every single LSAT. Ready? OK.
Dumbass argument one: Scientific studies have shown that people who got laid the night before tend to have more hangovers, on average, than people who did not get laid. Therefore getting laid causes hangovers.
The problem with this argument is, I hope, obvious. First, correlation doesn't prove causation. Second, specifically, how do we know that booze didn't cause both the getting laid and the hangover? There's an additional factor here that the argument simply ignores.
Dumbass argument two: Scientific studies have shown that people who eat poutine--that's french fries smothered in gravy and cheese curd, in case you've never been to Canada--tend to have more heart disease than people who do not eat poutine. Therefore, heart disease causes people to eat poutine.
The problem here is also, I hope, obvious. First, correlation doesn't prove causation. But specifically, how do we know that the poutine didn't cause the heart disease? Isn't that a more reasonable explanation?