According to Wanda, Maryann abused their magical powers. On the 28th of June 1914, she wished that Franz Ferdinand (Archduke of the Austro-Hungarian Empire) would be assassinated in Sarajevo, thus plunging the world into the First World War, which led to the rise of the Nazi Party, the Second World War and the Holocaust. It was for this reason that Cosmo and Wanda were taken away from her.
It is currently unknown how she was able to wish that the Archduke would be killed, because it states in Da Rules that a godchild cannot wish someone dead. However, it is possible that this rule was created because of her, or she found some sort of loop hole.
She was once wished out of Cosmo and Wanda's Hall of Infamy in their house by Timmy after he stole Cosmo's wand. She then tried to steal Wanda's wand and would have succeeded in getting her revenge, but was thwarted by Wanda, and put back into her painting where she belonged.