Our mother always said that making movies is like a box of chocolates. You never know what you're going to get. In the case of this Tom Hanks classic, it's a timing mistake that's out by more than a decade.
Forrest and Jenny have a fine old time in the film celebrating the 4th of July at Forrest's house, but were you keeping an eye on what was on television? His TV could apparently receive transmissions from the far future, because we can see a firework display at the Statue of Liberty... after it had been restored. Lady Liberty is looking her best in a way that wouldn't have been possible way back in 1976 because restoration work was only completed a full ten years later. Given that he was apparently getting news and pictures from days yet to come, it's no wonder that Forrest was a little confused about a few things!