2002’s A Very Muppet Christmas Movie is an “It’s a Wonderful Life” style movie where Kermit temporarily wishes himself out of existence and is allowed to observe the resulting timeline. Shots of the Manhattan skyline in the main universe where he exists show no Twin Towers as expected, but in the alternate timeline the towers are clearly visible in the background outside Piggy’s window at one point. Accident because the set designers or whoever just didn’t notice them in the old still used for that window, but the implication is hilarious