Via the Daily Dirt I encountered a farm (call it "Bob's") with a boat wanting three boxes of bread. I tried filling one box, which was allowed. I went to my farm to pick up more bread from the bread oven, and returned to Bob's farm. Trying to fill either of the other two then resulted in the bread dropping in but popping back out (without getting coins/XP/boatPoints, but also without losing bread). Semi-loading his farm by visiting Bob's fishing area and town did not change this behavior.

Minor nuisance for me, possibly more of a nuisance for Bob.