Explanation:
* This sentence has two independent clauses (complete sentences) joined only by a comma.
* This is incorrect because a comma is not strong enough to join two independent clauses.
How to fix it:
* Use a semicolon: The dog barked loudly; the cat hissed back.
* Use a coordinating conjunction: The dog barked loudly, and the cat hissed back.
* Make one clause dependent: When the dog barked loudly, the cat hissed back.