The Trap is Sprung Dragon Ball : Season 1 Episode 50

Treasure is supposedly hidden in the pirate’s cave. And to protect that treasure, naturally traps were set! Goku and the others somehow manage to pass through these traps, such as a wall that shoots spears, or a trap door that opens above lava. The Red Ribbon troops are completely wiped out, except for Blue!! Goku and the others come to the end of the path, and come out into the pirate’s sea-bottom dock. Goku can sense that something is moving. A skeleton-type battle robot appears, and immediately starts attacking them!!