Aha! The 13th labour and the only one he failed to complete successfully! From Wikipedia: "A traditional order of the labours found in the Bibliotheca[8] by Pseudo-Apollodorus is: Slay the Nemean lion. Slay the nine-headed Lernaean Hydra. Capture the Ceryneian Hind. Capture the Erymanthian Boar. Clean the Augean stables in a single day. Slay the Stymphalian birds. Capture the Cretan Bull. Steal the Mares of Diomedes. Obtain the girdle of Hippolyta, queen of the Amazons. Obtain the cattle of the three-bodied giant Geryon. Steal three of the golden apples of the Hesperides. Capture and bring back Cerberus. Dose the King's cat with a Worming tablet."