Though the time-line of the movie seems to last over several different days, every night is a full moon. It's possible it is all dream in a single night that might explain this.
When Russ is talking to his assistant about seeing a kid on his property, then goes to sleep he is wearing a blue t-shirt with UCLA on the top left and dark sweatpants, later when he gets up with the bat to check what made the noise outside, he's wearing a grey t-shirt and grey sweatpants.
Russ startles Rusty; Rusty throws his popcorn bowl and the popcorn scatters. When Russ spins the chair, the popcorn is not there, but when we see Rusty again, he and the chair are again covered with popcorn.
When Russ goes to Janet and asks her to "make little Rusty disappear", Rusty's hair changes from combed, to uncombed, and perfectly combed again.
When Rusty's father grabs him we see that he grabs his shirt, but in the next shot, his father is gripping Rusty by the shoulders. In addition, his father grabs his face shortly afterwards, but in the cuts back to Rusty, is still gripping him by the shoulders, until he rubs Rusty's eyes painfully.
Russ Duritz is supposedly 40 years of age. Bruce Willis turned 45 during filming. Spencer Breslin(Rusty Duritz) was 7.
The adult Russ Duritz and the child Rusty Duritz have different eye color. Bruce Willis has dark green eyes and Spencer Breslin has blue eyes. The color difference could be exaggerated by the lighting in the film, but eye color can change with age.
When Russ picks up the plane lying outside, the very bright spotlights where his car is parked made it appear like daytime to some viewers.
Russ and Rusty are supposed to be the same person however Bruce Willis (Russ) is left handed while Spencer Breslin (Rusty) is right handed. This is especially noticeable when Russ and Rusty are playing cards on the balcony but can also be seen when the characters are doing other things such as eating.
As Russ and Rusty are in the diner for the last time drinking chocolate shakes, they toast. When the glasses clink together you can see that Russ drops his drink but in a split second he has a full cup in his hand.
Bruce Willis and Spencer Breslin bear so little resemblance to each other that it is improbable that they could represent the same person at different ages.
The name of the diner is the Skyway Diner. But the credit for Melissa McCarthy says Sky King Waitress.
Towards the end when the Cessna is getting ready to leave with the 70 year old Russ as the pilot, he gets in the plane with the front wheel chocked. There is no one around to remove it so that the plane roll. Yet, it is removed.
While Russ confronts Rusty sitting in the chair, just after Russ turn off the TV, the boom operator is reflected in the T.V.