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.
Duplicate of MC-48399 - If you have not, please use the search function in the future, to see if your bug has already been submitted. If you could not find the original report, please comment with the keywords you searched for.