When you make a block update detector and you place a head, it doesn't detect that, but it does detect if you distroy the head.
Related issues
Comments

When you make a block update detector and you place a head, it doesn't detect that, but it does detect if you distroy the head.
migrated
(Unassigned)
Unconfirmed
blockupdate, head, redstone
Minecraft 1.5.2
Retrieved
Duplicate of MC-17047, please use the search function to see if your bug has already been submitted. Currently over 56% of tickets are being closed as duplicate.