Greek mythology
Iole was the daughter of Eurytus, who was a skilled archer. When the time came to marry off Iole, Eurytus had an archery contest (Grand Prize: Iole). However, Eurytus was a bit underhanded in doing this, as he was sure that either he or one of his sons would win. As luck would have it, Hercules showed up for the contest and won hands-down. Iole then decided that she didn't want to marry Hercules, because he was already married to Deianira. Eurytus sent Hercules away, refusing to allow him to wed Iole. Hercules (never one to take disappointment) returned later with an army, killed Eurytus and his sons, and took Iole as his wife.

Deianira took one look at Iole and realized that she had a bit of competition on her hands. Some time before, she had gotten some blood from the centaur Nessus, who told her that it would act as a love potion on Hercules. However, when Deianira put the blood on one of Hercules' shirts, the blood actually acted as a poison and killed Hercules.

Note: In the movie Hercules Unchained, Hercules' wife is Iole, with no mention of Deianira. I guess the above story wouldn't have made for a happy ending.

Dictionary of Classical Mythology