The phrase "careless mistakes" refers to errors or blunders that happen because someone didn't pay proper attention or didn't take enough care. Full definition
Potential employees, who make careless mistakes in application materials such as resumes, do not warrant the attention that a more careful candidate deserves. (thebalance.com)
Making careless mistakes on your cover letter will kill your chances of securing an interview. (zipjob.com)
Or, you may race through a project just to get it done and make a lot of careless mistakes. (health.com)