Crashing when I move in particular areas of the world: RuntimeException: Missing BlockEntity

Only happens when I move through certain parts of the world. The chunks I moved through were generated a long time ago (Alpha 1.2.1_01). Game mode doesn't make any difference. Doesn't affect other players on the server (who are in newer chunks).

Each time it crashes, I get: minecraft:flower_pot[contents=cactus,legacy_data=2]

Full crash dump attached.