I'm not sure this is a "bug", but atleast it's something that i'm sure most people would like to see changed:

When you are using the new execute-command, with a player as it's entity, it count's as if the player did it. This means, that even if you have CommandBlockOutput set to false, you still get the "Block placed" or "Object succesfully summoned". If this should be used in a adventuremap, that text has to be removed.