A merchant who owned a donkey heard that salt was cheaper by the seashore, so he decided to go into the salt business. He went and loaded his donkey with salt and then headed back home. At a certain moment, the donkey accidentally lost his footing and fell straight into a stream. This caused the salt to dissolve, making his load lighter. The donkey was thus able to rise easily to his feet and enjoy a less taxing journey home. The merchant sold what was left of the salt and led the donkey back again to load him with an even greater cargo than before. As the donkey made his way with difficulty back to the stream where he had fallen before, he sank to his knees on purpose this time. Then, after his cargo had dissolved in the water, he leaped nimbly to his feet, delighted to have turned the situation to his advantage, or so he thought. The merchant realized what was happening and decided that the next time he would bring back home a big load of porous sponges. On their way back across the stream, the wicked donkey fell down on purpose as before. This time the sponges grew heavy with water and the cargo expanded. As a result, the donkey had to carry a burden that was twice as heavy as it had been to begin with.
Source | Aesop’s Fables. A new translation by Laura Gibbs.
(Oxford University Press, 2002) page 78
Here’s a slightly different version
A Salt Merchant drove his Donkey to the seashore to buy salt. His road home lay across a stream into which his Donkey tripped and fell by accident. When the Donkey got out of the water, his load considerably lighter, as the water melted the salt in the sack.
The Merchant went back to the market by the seashore and refilled his bags with a larger quantity of salt than before. When he came again to the stream, the Donkey fell down on purpose in the same spot, and, regaining his feet with the weight of his load much diminished, brayed triumphantly as if he had obtained what he desired.
The Merchant saw through this trick and drove the Donkey for the third time to the coast, where he bought a cargo of sponges instead of salt. The Donkey, again playing the fool, fell down on purpose when he reached the stream, but the sponges became swollen with water, greatly increasing his load. Thus his trick recoiled on him, for he now carried on his back a double burden.
It often happens that the same things which brought us luck can also get us into trouble.