The verbs are consistan. As a habit or routine is being described,both verbs go into Simple Present.
Answer:
It contains a comma splice.
Explanation:
"Don't stack twelve boxes on top of one another" and "it's the twelfth one that will knock down the pile" are independent clauses and should be linked by a colon, semicolon, or conjunction.
"The alternative—in vitro toxicity testing—involves tests conducted on cultured bacteria or mammalian cells in a lab."
It tured humourous like he was a stand up comedian
