Continuity mistake: As the boys are running through the sewer tunnel, both of them fall several times, and they are covered with dirt and sewage. When they come out and face the bullies, their clothes are clean.
Continuity mistake: At the end when Mike receives the first postcard from Bobby, the flag on the mailbox is up, meaning there is mail to be picked up by the postman. Then when he takes the mail out and runs towards the house, the flag is down without him touching it.
Continuity mistake: In the first scene with Bobby's first postcard, he writes his name as "doddy" as a young child might, reversing the Bs. Later, when you see the postcard hanging by a string, it is written "bobby" instead.
In the scene where Bobby and Mike go into the comic book store, you can clearly see a Transformers comic on the shelf. But the movie is set in the late 1960's/early 1970's....and Transformers did not come out until the 1980's.