comma splice
Joining two independent clauses with only a comma, which isn't strong enough for the job.
A comma splice occurs when a comma alone joins two independent clauses. A comma is too light a mark to glue two complete sentences together by itself; the fix is a period, a semicolon, a comma plus a coordinating conjunction, or a semicolon plus a conjunctive adverb.
Examples
The old clock looked ancient, it had a cracked stone face.