There are potential red flags to avoid mentioning in the letter. If the applicant has a criminal history, it's generally best to avoid mentioning it in the letter, as it could raise concerns about the applicant's suitability for immigration status. Any mention of the applicant's involvement in illegal activities, such as working without authorization or overstaying a visa, should be avoided. Financial issues, such as unpaid debts or outstanding taxes, could raise concerns about the applicant's ability to support themselves. Health concerns may raise concerns about the applicant's ability to support themselves and their potential burden on the healthcare system.