Lazy, Grouchy, and Smurfette wade into the swamp to retrieve the Lilly roots. When they encounter the Custodian, the shore along the water changes in the shot of Lazy, Grouchy, and Smurfette and the Custodian and they Smurfs are also closer to the shore.
When Lazy, Grouchy, and Smurfette get stuck in the swamp during their first encounter with the Custodian, Grouchy is positioned between Lazy and Smurfette. When the water turns to quicksand and they start sinking, Grouchy and Lazy have switched places. Later, when the Custodian walks up to them to ask if they have any last requests, Lazy and Grouchy have changed spots again. Then when the sand is up to their noses, they switched places again.
After the swamp turns to quicksand, Grouchy, Lazy, and Smurfette sink down until the sand is up to their necks. Later, when Smurfette yells at the Custodian, the sand is up to their lower chests. Then when the Custodian walks up to them and asks if they have any last requests, the sand is back up to their necks.