LOL! I always forget something when we travel, but its never the same thing twice. Some common forgotten items: hair dryer, hair brush, razor, toothbrush/toothpaste, alarm clock (not all hotels have them), camera equipment/film/batteries, hats. Its usually either a last minute packing item (something that is in use until the day of departure) or an off-season item (like a hat during a winter trip).
Steve