I was harvesting my lettuce, with a scythe, and around 3-5 of the lettuce wouldn't harvest. The bar would complete, then nothing would happen, it'd just stay there. And for some reason, taking off my scythe and doing it by hand worked.
EDIT: Happened to a single patch of my poppys too, but this time simply right-clicking and clicking harvest worked with the scythe, unlike the lettuce, which did nothing with RMB and harvesting, or drag-harvesting.