Villagers generated in older versions will trade carved pumpkins instead of normal ones in the 1.13 snapshots. Newly generated villagers, however will trade normal, uncarved pumpkins.
- Create a world in 1.12 and spawn a farmer villager
- Do one trade so it unlocks his pumpkin trade
→ Note that the villager will trade pumpkins (only kind there is)
- Open the world in the snapshot (18w20c)
→ Note that the villager will now trade carved pumpkins and won't accept normal pumpkins
- Spawn a new farmer and do a trade
→ Note that this new villager will trade normal (uncarved) pumpkins and won't accept the carved ones