You all know this rule: Cardinal Rule No. 1: Put a comma before a coordinate conjunction when it is followed by an independent subject and verb.
But be careful of the dependent clause that LOOKS LIKE an independent clause.
…You have to be careful that you have checked out all the details and you know the pitfalls of the deal.
…She claimed to always be late because she had to drop her kids at her mother’s and her mother always had an issue or two.
In the second part of this compound DEPENDENT construction, the introductory word for the clause is omitted, leaving the clause looking as if it is an independent subject and verb. You have to check back to the beginning of the sentence to see what is going on.