Continuity mistake: When Max is going to get K.W. just prior to the Dirt Clod War, his hood is down. When he returns to Carol and Douglas, (just prior to being hit with a dirt clod) his hood is up.
Continuity mistake: After Max runs away from home, just before he jumps onto the boat, he slides through the dirt and even a big bunch of dirt can be settled on his right shoulder. When he jumps on the boat, less than a second afterwards, the pajama is spotless.
Continuity mistake: The first morning, when Carol is introducing the creatures to Max, look at Carol's nose and its diagonal scratches. The scene is mirrored from time to time.
Continuity mistake: On almost every single angle of the scenes on the island, Max's hairstyle is inconsistent between one angle and the other, in a matter of nano-seconds. It keeps swapping from messy to brushed, from dry to wet, from clean to dirty. It's an absolute chaos.
Continuity mistake: The first morning on the island Max is by the cliff and he has a black spot on the right side of his nose that keeps appearing and disappearing depending on the angle.
Continuity mistake: When the monster takes Max out of its stomach, he is all covered in slime and big chunks of saliva. Half a second later, it's all gone and he is just slightly wet.
Chosen answer: In the book it was just his imagination; he was gone for years in his mind but only a an hour or so in real life. It is presumably the same basic idea in the movie.