Affects Version/s: Minecraft 14w29b
Fix Version/s: Minecraft 14w30a
Linux, Java version 1.7.0_55
A chunk in a map created with an older version of Minecraft, when loaded into a current snapshot, may be wiped and regenerated if it contains a painting.
The attached world save is derived from a custom map made in Minecraft 1.2.5, "Minecraft à la Portal (German)". (http://www.minecraftforum.net/forums/mapping-and-modding/maps/1489138-minecraft-a-la-portal-1-2-5) It's not my map; it's just the one I used to isolate the problem.
Load the attached save in 1.7.10, and the player will spawn next to a normal-looking chunk that contains a painting. Load the attached save into a current snapshot instead, and the player will spawn surrounded by empty space. The entire chunk is erased upon load and replaced by a new (empty) chunk.
The attached world save is just for convenience in reproducing the problem. The problem also occurs when loading the original world into a current snapshot. If you try this, the spawn chunk will clearly be wrong--the player will fall out of a structure onto terrain that's a different biome from the surrounding terrain. Other nearby chunks will also have been erased and regenerated. If the world is loaded into 1.7.10 instead, those missing chunks will be present.
I'm not an expert in the file format, but the problem seems to be with the "Dir" field of the painting's entity tag. Using pymclevel to rename the field to "Direction" fixes the problem.
Loading the map into 1.7.10 fixes the problem. That is, a map that has been loaded into 1.7.10 and saved can then be loaded into a snapshot without problems.
The problem seems to have been introduced in 14w20b and is still present as of 14w29b.
Since item frames can also have the 'Dir' field, it's possible that chunks containing item frames are also at risk of being wiped.