The example in the title: while in build mode pressing and holding the Tab key and clicking to sort on any of the columns (faction name for example) will place a block on the surface behind the scoreboard.
Similar issue:
When re-sizing UI elements using the drag controls in build mode, sometimes a line of blocks is placed behind that UI element (where you dragged the mouse)
Basically, interacting with UI elements does not always block the player from executing unintended actions in the game while in build mode