Aphrodite did not very much like being married to Hephaestus. She was the goddess of beauty, and he was uglier than a stone. She was so dissatisfied that she cheated on her husband with a host of other gods, most of all Ares.
Hephaestus found out about their affairs and set a trap in the bed. When they got in the bed, the trap sprung. It was a net that completely encompassed them, preventing them from moving at all. However, it was also perfectly transparent.
Hephaestus hung the two, still in the net, from a stake in the center of Olympus for all to see.