I can completely understand your frustration. While this particular thing hasn't happened to me, something similar has. I've put a villager on an herb to put in the stew and I go check on other villagers. When I put another villager on the stew to add water/food, it says I still need an herb (and I've seen the other two herbs being put in). Apparently the third I put to get the herb got bored and neglected their duties to go off and do something else LOL. The villagers have a mind of their own. They don't always do what you tell them to LOL.