The bug

You can't see the blockdata of a flower pot change after using a /blockdata command.

How to reproduce

1. Place down an empty flower pot
2. Use /blockdata x y z {Item:"cactus"}
   → Nothing happens
3. Press F3+A (to reload all chunks), or force a block update, or log out and log back in
   → Now the flower pot has a cactus in it

Why this should be fixed

If a custom map maker has a flower pot on a piston head (a block that will disappear if there is a block update) then setblock would not be an option since the piston head will disappear. Blockdata doesn't make the piston head disappear but it does require a client-side refresh.

Code analysis

Code analysis by marcono1234 can be found in this comment.