That vs. Which: Spot the Mistake on Yahoo

Mark says:

I was poking around in Yahoo's help section, and I ran across a grammatical mistake in this FAQ. Can you spot it?

Nice try, Mark. But you'll have to up your game if you want to stump the Snarkmeister. 

Where does Yahoo's FAQ go wrong? It uses which instead of that to introduce an essential clause (a.k.a. restrictive clause). 

Quick rules: 

• Use that to introduce an essential clause. The clause is necessary because it specifies the noun in question. If you removed the clause from the sentence, you would change its meaning. I want you to fix the bikes that have wobbly seats. Which bikes? Only the ones with wobbly seats.
• Use which to introduce a non-essential clause. The clause could be removed from the sentence without changing its meaning. I want you to fix the old bikes, which have wobbly seats. In this case, the essential description of the noun 'bikes' comes from an adjective. Which bikes? The old bikes. The phrase 'which have wobbly seats' adds detail but doesn't change the meaning of the sentence. You could simply say: I want you to fix the old bikes.
• Always use a comma with which. Don't use a comma with that.

Nifty Mnemonic:

• An old copy editor's saying: That defines, which describes.

Let's go back to Mark's example.

What should I do if I find content which is illegal or violates the Yahoo! Terms of Service?

Yahoo rightfully does not put a comma after the word 'content' and, intuitively, we do not pause there. The lack of a comma is a huge, honking clue that we need that and not which.  But more importantly, the person asking this question isn't wondering what to do with funny content or boring content; the FAQ is singling out illegal content. That makes the clause essential to the sentence, so we must use that.

Correction: What should I do if I find content that is illegal or violates the Yahoo! Terms of Service?