<em>Yes</em>, when a problem is severe or harmful, scientists are ethically and legally obliged to speak out.
Scientific experiments and testing are vital processes that lead to innovations, medicines, procedures, etc., that better human life. While scientific testing is required before a medicine, product or procedure can implemented or introduced in the market, it is essential to stop or prevent further testing when a severe problem or harm arises at the experimental stage.
For instance, if a clinical experiment testing the effects of a new hair loss prevention drug finds that consumption of said drug causes severe nausea and dehydration, scientists must speak out or stop the experiment to prevent further harm. A failure to prevent harm is both highly unethical and liable to legal consequences.
