chol hamoed - How do you violate the intermediate days of the festivals?

The Talmud (Sanhendrin 99a) states:

He who defiles sacred objects, he who violates the intermediate days of the festivals, or he who embarrasses another person in public, even if they are learned in Torah and have done many good deeds, lose their portion in the eternal world.

Defiling sacred objects and embarassing others is pretty clear, but how exactly do you violate Hol Hamoed (intermediate days of the festivals)? The primary days of the festivals have the 39 melachot (except cooking), but the melachot don't apply during Chol Hamoed!