This is a link to a study of stem cells that can develop abnormalities if grown over months.
http://www.theaustralian.com.au/higher-education/stem-cells-vulnerable-to-cancer/story-e6frgcjx-1226207714863?from=hot-topics-he
http://www.theaustralian.com.au/higher-education/stem-cells-vulnerable-to-cancer/story-e6frgcjx-1226207714863?from=hot-topics-he