I was messing around with villagers in creative mode, and creating some with custom trades. I wanted to make a villager that cannot be traded with, so I used this command:

/summon Villager ~ ~1 ~ {Offers:{Recipes:[{maxUses:1,buy:{id:,Count:1},sell:{id:,Count:1}}]},Profession:3,CustomName:"Npc1",CustomNameVisible:1}

This command summons a proper villager with blank slots in its trade menu. However, closing and reopening the world causes the chunk the villager is in to revert to what it was before the villager was spawned. Summoning a villager with actual item IDs will not cause this error. For example:

/summon Villager ~ ~1 ~ {Offers:{Recipes:[{maxUses:1,buy:{id:"minecraft:barrier",Count:1},sell:{id:"minecraft:barrier",Count:1}}]},Profession:3,CustomName:"Npc1",CustomNameVisible:1}

Although villagers with blank trades was probably not intended to exist, I think that this bug could use to be fixed just to make stuff look better... Without losing tons of work in one chunk.