The handwriting on the envelope when Ronnie's Mum is writing
her dying note to him differs (twice) from the original shot. It is seen three times: first when she is writing it (the N's are quite distinctive), second when he discovers the note in the case (it is clearly different) and thirdly after he has washed up and sits down to read it (it is different yet again). Also the handwriting on the envelope that Brad writes to Kathy differs between its two appearances. The first shot shows all capitals ("KATHY") while the second shot does not ("Kathy").
In the hospital, Ronnie's mother writes the name Ronnie on an envelope. When Ronnie opens the envelope later at home, the writing is slightly different.
When Sarah is running out of the park calling Lucy's name, her cardigan is barely falling off of her shoulders. When she stops and screams Lucy's name, she is shown to her chest and no cardigan is seen. Next shot, she's running to Lucy wearing her cardigan again.
When Brad is dropped at the train station, the train has "Long Island Rail Road" on the side. When Brad is picked up at the train station the station sign says "Boston", and the LIRR logo has been covered. However, the LED Destination sign on the side of the train says "Penn Station" which is the Long Island Rail Road terminal in New York City.
The envelope from the credit card company has "08945" (non existent) as the East Wyndam Zip Code. The bar code does not decode to numbers. When Mom takes out the classified ad for Ronnie, the East Wyndam Zip Code is now 02748 (South Dartmouth, MA). The return-address stickers that mom puts on the after-death note to Ronnie has a East Wyndam Zip code of 02786 (non-existent).
Camera reflection on the car when Kathy picks up Brad at the train station.
When Larry references the mall shooting he says the boys were playing with an air gun at the Big 5. Big 5 Sporting Goods stores do not exist east of Texas, while the story takes place in Massachusetts.
Ronnie is said to be on probation for indecent exposure to a child. There is no way he would be allowed at the town pool while children are present. It is a given that his probation would include him being required to stay a certain distance from minors.
On the sunglasses of the woman by the pool who declines her children a snack.