Career-homicide is when people ruin your chances at getting a future job or internship by using your urbandictionary account to add inappropriate/highly offensive material that can be searched via Google and then potential employers Google your name and find that offensive material, and then automatically assume you wrote it and it ruins your chances at all possible future employment with any reputable organization. Other examples include people posting unflattering pictures of you on the internet/Facebook, tagging you in inappropriate Youtube videos, etc.
Some jerk commits career-homicide by using your urbandictionary account several years ago thinking he is being funny and posts multiple offensive definitions on your account. You realize what has happened, spend hours as an urbandictionary editor attempting to get the definitions removed the site, spend hours trying to contact urbandictionary.com executives to get said definitions removed, and eventually fail to have them removed because they are "too popular". Then, years later when you start applying for jobs and internships, the companies you apply to Google your name, see what they assume you wrote even though it wasn't you, and then proceed to quickly throw your application into the trash, all because some jerk ruined your good name through trying to be funny. And eventually you realize that someone has "murdered your career" via career-homicide on UrbanDictionary.com.