At the beginning of the final barbecue scene, Wally is seen using a gas grill, as evident by the burner knobs. After walking in the house, he speaks of charcoal and lighter fluid, which are completely unnecessary when using a gas grill.
In the beginning of the film, Wally and Kassie are seen taking the A train. Although we are not shown if they switch trains, when she exits the train and the doors are closing, we see they are now aboard a 2 train.
When Wally and Sebastian are walking through the park the woman jogging and the man on the bike with the beige jacket can both be seen crossing the camera twice in under 30 seconds. Without doubling back this is not possible.
During the climbing wall/birthday party scene, Roland and Sebastian are both on their own belay ropes when Roland goes up the wall to get Sebastian. When Roland is lowered to the ground, Sebastian's rope is no longer there.
When Kassie suggests to Wally on the phone that he bring Sebastian a picture frame as a gift, Wally takes one off his desk and removes the picture from it to give to Sebastian that night -- but Sebastian is disappointed because he knows it is used. Later, when at the frame shop to get Sebastian a new one, Wally brings in the old one with (apparently) a receipt, chooses a new frame, and tells the clerk he's exchanging the old one for the new one.
Sebastian has brown eyes, while Wally and Kassie both have blue eyes. Two people with blue eyes cannot have a brown-eyed child (since both parents would have to have two copies of the recessive blue-eye chromosome, which would get passed to the child). Sebastian cannot be Wally's son.
The goof item below may give away important plot points.
During the final scene(s), Wally is at the BBQ wearing his wedding ring on his left hand but when he walks into the kitchen (continuous shot), it is now on his right hand, as is Kassie's. Then when they both walk over to the table with the cake, their rings are back on their left hands.