This happens because HeLa cells are very hardy, robust growers and they're found in most tissue-culture labs. So if you get even a single HeLa cell in your culture, they'll eventually proliferate and take-over the cells you thought you had. Regardless, there are still many, many other legit cell-lines.
http://www.nature.com/news/2010/100602/full/465537a.html
This happens because HeLa cells are very hardy, robust growers and they're found in most tissue-culture labs. So if you get even a single HeLa cell in your culture, they'll eventually proliferate and take-over the cells you thought you had. Regardless, there are still many, many other legit cell-lines.