Margot Woelk, 95, was one of a dozen women whose job was to taste the food prepared for Adolf Hitler to detect poisons. She recently descibed to reporters her life at the Wolf's Lair, Hitler's headquarters facility in eastern Poland:
She said: “Between 11 and 12 o’clock, we had to taste the food, and only after all of us had tried it was it driven to the headquarters by the SS.
“It was all vegetarian, the most delicious fresh things, from asparagus to peppers and peas, served with rice and salads. It was all arranged on one plate, just as it was served to him. There was no meat and I do not remember any fish.
“Of course I was afraid. If it had been poisoned I would not be here today. We were forced to eat it, we had no choice.”
At the link, you can read descriptions of Hitler's poor table manners and Woelk's escape from advancing Soviet troops.
Take botulinum toxin, for example. It acts between 18 to 36 hours to manifest its symptoms after intake. Food tasters would've been useless in this case.