- Tiles are written concurrently, on the assumption that each thread works on a different set of blocks - This assumption fails when a free blob at the end of a segment is reallocated, in which case the `prev_blob_free` flag of the *next* blob is cleared - A thread writing tile data into this next blob potentially resets this flag because it read an earlier version of the blob header - **Solution**: When writing a tile, *never* write the blob header; let `BlobStore` handle this exclusively