From the Super Mario Wiki
In The Super Mario Bros. Super Show! episode "Mario and Joliet", Grunt, after helping King Koopa put Mario, Luigi, Toad, Princess Toadstool and Romano in a prison cell in his castle, is charged by King Koopa with the responsibility of guarding them and was given a key to the cell. Apparently ignoring his guarding his responsibility of guarding the captured Mario, Luigi, Toad, Princess Toadstool, Romano and Joliet, Grunt begin to lift a nearby set of weights.
Seeing Grunt lifting his weights through the barred window of his cell, Toad got an idea and began to ridicule Grunt's strength, saying he had seen a Koopa Troopa lift twice as much as Grunt. Grunt, wanting to prove his strength, lifted another heavy set of weights in one hand, after placing the weight he had lifted earlier in his other hand. Pretending to be unimpressed by Grunt's display of strength, Princess Toadstool tricked Grunt into lifting another weight in his mouth. It was after Grunt lifted this third weight with his mouth that the floor beneath him, being unable to support the combined weight of Grunt and his exercise equipment, collapsed, sending Grunt plummeting downward.