Depends on the food really. For example, a lot of places when they get lettuce, the lettuce will have a couple of small black insects in it. Though the food preparers will usually try to get rid of the insects they might find, they're still only human. If you do find one, you can either get a free replacement meal OR a refund. Also, at least in the United States, most restaurants are checked by the health department regularly for such things. So if you did get a bug in your food, in most places, it's probably a rare occurance.
From personal experience, lettuce is very easy to wash. And its color is very easily distinguishable from black. In comparison, some dark green vegetables are really hard to wash and insects can much more easily camouflage in there. Anyone with good eyesight who puts a little effort should easily be able to remove insects from lettuce, but I can understand the troubles with dark green vegetables.
I remember the time my mom brought home some dark green vegetables(don't know the name, I only know my vegetable names in Chinese) for me to wash that her friend, who planted the veggies herself, gave her. She didn't use pesticides and stuff, so... I was in for some nasty surprises when washing them: two live worms and one live large mosquito. I got so freaked out I threw out the whole batch.
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I don't understand why some people like to go out for a picnic, allowing insects to fly into their food.
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In yesterday's packed lunch from the Chinese restaurant which contained egg+tomato, I bit into something hard. Spitting it out revealed small pieces of egg shell.