Once, I went to a restaurant and almost the entire menu was different than the website had when I checked the night before. We hit the day that they had switched the seasonal menu. It was perfectly okay with us, just meant we had to look closer. I noticed that most items were the same/similar with just changes to sides or sauces. I think the type of fish had changed for the season.
I have noticed that menus might miss some items, but usually not the main dishes, except for specials for a day or short time.