Other mistake: Josh's birthday, according to the milk carton, is in the winter. But, some time after he celebrates his birthday at the Italian restaurant, he returns to his childhood neighborhood and it's fall (colored leaves, etc.). It can't be the following year because he's only gone for a total of a few weeks.
Other mistake: When Josh and Billy are over the balcony dropping the water balloon on the mover on the street, the balloon is shown dropping from an overhead angle off course from hitting the man below, yet we hear him scream being hit, when it easily would have missed him where Josh dropped it.
Answer: When Josh started at the toy company, he torpedoed Paul's skyscraper transformer by saying 'I don't get it.' Paul is returning the favor.