Blocks within the world have been showing "black and purple" checkerboard textures - the following blocks have been affected:

Stone and Grass blocks.

This appears to be occurring to random blocks/random selections of blocks - I have checked the following versions and they all appear to be affected:

Version 14w19a, 14w18b, 14w17a, 14w11b

Version 1.7.9 is NOT affected.

The blocks themselves when analysed in MCEdit show the following ID's: 1:9 (Stone blocks which were affected) and 2:8 (grass blocks affected) - they also show as being "null" models and flash up as "Round trip different" when the F3 console is open and the blocks are looked at.

Middle-clicking on the blocks produces an ordinary block which if used to replace the glitched blocks makes them appear "normal" again - the blocks are also easily changed through MCedit by replacing the block ID's with normal, expected block ID's (e.g. replacing 1:9 blocks with 1:0 blocks resolves the issue.)

.olding the .Minecraft folder in the "Roaming" folder has had no effect and the issue is experienced with both the normal resource pack along with additional resource packs that are added.

The map has also been updated from an earlier version - the map itself has been around since before the 1.7 update - the areas experiencing the issues were also loaded in before the 1.7 update. (e.g. the map has been periodically moved from update to update as the new snapshots and versions have been released.)