Explanation:
* Comma splice: The sentence incorrectly uses a comma to join two independent clauses (complete sentences) without a coordinating conjunction (and, but, or, nor, for, so, yet) or a semicolon.
* Independent clauses: "The cat sat on the mat" and "It was a very comfortable mat" are both complete sentences with a subject and a verb.
How to fix it:
* Use a semicolon: The cat sat on the mat; it was a very comfortable mat.
* Use a coordinating conjunction with a comma: The cat sat on the mat, and it was a very comfortable mat.
* Create two separate sentences: The cat sat on the mat. It was a very comfortable mat.