When you place a barrier block in creative mode it shows the barrier block icon ⃠ floating in the middle of the block. After a minute or so it goes away and the block acts as it should (invisible). When you place another barrier block it happens again, but it happens with with every barrier block that has been laid. This does not happen in survival, adventure or spectator mode, only creative. If you switch to one of those gamemodes while the ⃠ icon is visible it will disappear.