The Crab

In the ancient tellings, mid-labour, with Heracles already struggling, a giant crab comes out of the marsh to help the Hydra by nipping at his feet. He crushes it. It is such a small, absurd intervention that it would be easy to read as comic — except for what happens next: Hera puts the crab in the sky as Cancer, as her reward for the attempt. The episode is a precise piece of characterisation for a goddess who does not forget service and does not forgive Heracles. The labour was later disallowed for another reason: Heracles had help from Iolaus, so Eurystheus refused to count it.