When Eric is talking to Mr. Feeney about entertaining his niece, the size of the piece of pizza he's eating changes, and he takes a bite twice between cuts.
When Eric first meets Mr. Feeny's niece, he throws a bag of mulch. When the mulch lands, you can see the fake grass move, and the fake background swings a little bit.
When Alan is telling the rocket story to Cory, he mentions the Franklin Space Museum. The real place in Philadelphia is actually called "The Franklin Institute".