At the rate of speed the piano moved through Fred's house, the piano should have been already on the street by the time Fred explained to Wilma that what she saw she didn't see. About 40 seconds later, Barney, laying on the ground, tells Fred he has just been run over by a piano, and then the piano is shown just beginning to leave Fred's yard.
Fred lets go of the rope while trying to lift the piano up to a window. The piano falls on Barney. But neither of them suffer any damage/injuries.
When Wilma gives Fred the heart shaped toast while wearing the wedding veil her eyes change alternating back and forth from solid black pupils to pupils with whites.
Fred's car is shown with a back seat as a sedan when normally it's a two seat coupe.
After Barney screams into a mailbox, the mailman comes to collect the mail and lets Barney's scream out of the mailbox. However, it's after midnight when this happens, a bit late for the mailman to be picking up the mail.