Error hit when deleting blocks in coloring-blocks example

[diarmidmackenzie]

Open the coloring-blocks example, and right click quickly to delete a series of blocks.

In the console it's fairly easy to generate this error:

instanced-mesh.js:793 Member block-5-7-7 not found for modification

Suspect some sort of face condition, but haven't investigated. Could be an issue with the example code, or perhaps an issue with the instanced-mesh code itself.