I've been having trouble setting up GUI's using button sprites that don't conform to the standard rectangular shape of the GUI buttons.
In some cases, the GUI buttons can overlap due to their square shape because of buttons being aligned in a diagonal direction.
If it possible to set up a GUI so that the transparent segments of a button sprite are non-clickable?
A forum search for "buttons transparent" would have brought up this workaround (http://www.adventuregamestudio.co.uk/yabb/index.php?topic=28882.0) which I came up with a while ago. Make sure to read the part below the code about overlapping buttons.