When Luca drops and ice-cream near the fountain - it disappears merely few moments later.
When Signora Marsigliese declares that the bike has entirely crossed the finish line, the bike is only just across. However, in a later shot it is much farther away.
Massimo declares Alberto and Luca the winners and lifts Alberto up. However, since Luca is racing alone Alberto would still be part of Giulia's team, who didn't win.
When radio falls off Ercole's Vespa - it disappears in next shots.
While swimming to Portorosso, Luca and Alberto jump out of the water several times while briefly turning human. The drops that are still on them magically move away even though they don't shake them off. And if they're able to control water this way, they could just use those powers instead of drying themselves off whenever they accidentally come into contact with water in Portorosso.
In the scene where Luca and Alberto are asking Giulia if they could turn their prize money into a Vespa, the price tag of the beaten-up blue scooter is 9000 lire. The currency symbol on the price tag in the scene is a British Pound (£), which has one strike-through. The symbol for the Italian lira is looks similar, but it has two strike-throughs instead of one.
While Luca is walking through Portorosso for the first time and the sun disappears behind the clothesline, the shadow can be seen across Luca's face, completely in-line with the shadow on the ground. Luca's head's shadow is well outside the clothesline's shadow, so his face should not be shaded.
In the after-credits scene, Ugo is back in his natural habitat and can be seen talking to a fish that would not be able to survive at such depths.
Since no one saw Luca swim around the buoy, he should not have been allowed to move on to the eating round.
Luca would have been disqualified for racing with Alberto, who is not part of his team.
Daniela and Lorenzo are trying to find Luca to avoid he could be exposed, and they do this by splashing everyone they suspect is him with water. They probably didn't think this through because if they actually succeeded, Luca would end up exposed in front of everyone.
When Alberto is running in the rain with the giant blue umbrella his Bare feet are not changing back to their true form despite his feet touching the wet/rain soaked road.
When Luca's parents first get out of the water they don't know what they are looking for, they've never seen humans before, but they instantly know how to walk, and when they first spot a human they run and jump behind a rock without actually learning how to do any of this as Luca did in the start.