In the Twelve Labors of Hercules, Eurystheus originally assigned ten labors before two were rejected.
The cycle began with ten tasks imposed on Hercules as penance after he killed his wife and children during a divinely caused madness. Hercules completed the Nemean Lion, Lernaean Hydra, Erymanthian Boar, Ceryneian Hind, Augean Stables, Stymphalian Birds, Cretan Bull, Mares of Diomedes, Belt of Hippolyta, and Cattle of Geryon.
Eurystheus refused to count the Hydra because Hercules had received help from his nephew Iolaus. He also rejected the cleaning of the Augean stables because Hercules had expected payment, or because he had used the rivers’ water. Two additional tasks were then added: obtaining the apples of the Hesperides and capturing Cerberus.
That accounting explains why the familiar expression is “the Twelve Labors,” even though the earliest assignment in the standard narrative contained ten. Ancient sources do not present every detail identically, so lists and ordering can vary.