Oh, I have so many of these. Fifty years as a vegetarian has given me a long list. Many of which share a theme with several PPs' stories.
Hospital: not a worst meal but a compete lack of meals. Monday admitted for surgery, nil by mouth; Tuesday, in surgery so missed the menu rounds and all meals; Wednesday, having not filled in the menu choices the previous day I was presented with whatever random bits discharged patients had ordered - but could eat none of them, given menu choices for Thursday and they had nothing suitable. I wasn't allowed to leave until I'd eaten something, but fortunately they went by self report otherwise I'd still be there.
Planes: British Airways transatlantic flight gave me the fruit platter with side of fruit salad mentioned by a PP. Breakfast was the same fruit salad again. Which was at least better than Delta transatlantic, which gave me dinner of a packet of chicken crisps (made with actual chicken) and a dry bread roll. No breakfast. A Bulgarian airline provided plain white rice and frozen peas - with a side of food poisoning.
Russia: everyone else had meat pierogi, I was given a raw papper sliced in half and filled to the brim with unsalted butter. And a meat pierogi.
Greece: told them I was vegetarian, double checked the cheese pizza was suitable. It arrived with a layer of ham under the cheese, and they were unable to comprehend the problem.
Bulgaria: vegetable soup that was 1 slice of raw tomato and 4 slices of raw carrot in the bottom of a bowl, barely covered with hot stock, then filled to the brim with hot sunflower oil.
France: fancy, multicourse, catered dinner. As is common in France each item was served as a separate course, so when it came to the meat course there were no side vegetables - those were to follow as the next stage. Everyone else had a piece of meat with mushroom sauce and a parsely garnish. I had a plate with a tablespoon of sauce and a sprig of parsley.
Spain: cold garlic soup, which was just slightly diluted garlic puree.
Austria: fried cheese served with a cottage cheese dip, followed by cheese spätzle, followed by cheesecake, finishing with a cheeseboard. Doesn't entirely fit in the list of 'worst meals' because it was all really good food, but a badly planned overall meal because it was Too Much Cheese.
UK: too many to list, but a particularly memorable one was a restaurant with the tagine "healthy food doesn't have to be boring" and prided itself on a wide selection of vegetarian options. The 'spicy beanburger' was red kidney beans. Just red kidney beans, squashed together - no binders, spices, not so much as a pinch of salt. It came avocado beautifully presented in artful spirals. Which looked very pretty, but if you can slice avocado with a vegetable peeler and then curl the slices, that's a sign it's at least a week away from being ripe. Healthy food doesn't have to be boring, but theirs certainly was.