Parenthood
In some cases, health issues are the reason for a sudden change. The actress Maura Tierney found out that she had developed breast cancer just prior to filming her role as Sarah Braverman on Parenthood. Lauren Graham went on to replace her and even married co-star Peter Krause. Best of all, Maura was able to overcome and beat cancer for good!
Shameless
Mandy Milkovich in Shameless is just one of the many deranged and upsetting characters from this highly unhinged drama. During the first few episodes of the starting season, Mandy was played by actress Jane Levy. However, Jane left the program shortly after starting to star in the acclaimed comedy Suburgatory. Actress Emma Greenwell played Mandy for the rest of the show until she was written out in the sixth season.