Block spawners, such as sand entity spawners etc., cannot spawn sand entities which turn into block 36.
This is specifically a problem for mapmakers since block 36 has very unique properties and can be made solid or pass-able. For example you can make walls which appear to be invisible,are solid and mobs still track players through this wall as if it wasn't there.
To recreate the issue easily:
1. Download MCEdit and create a region of block 36 blocks.
2. Next use http://sethbling.com/structurespawner2 to create structure spawners for the region of block 36.
3. Next delete the region of block 36.
4. Go into minecraft and try and spawn in the structure with the structure spawning.
The first image shows what happens when you try and spawn the block 36 in.
The second image shows one of it's uses. A 2 by 2 region of block 36, which is to turn into iron blocks in 99999999999 seconds, is used to hold back a horde of zombies tracking the player while the player can shoot through the region with a bow and hit the zombies with his diamond sword.
All this having been said we can destroy the region of block 36 with the spawner method.