I have a three-button button (where you can left, middle, and right click) where the left-click is set as a menu.
If I alt-click the button itself I get the editor window and can successfully make changes.
If, however, I left-click the button and then alt-click one of the menu items I get the editor window but any changes I make are not saved when I click OK.
Has anyone else experienced this problem?
Putting menus in the left-button slot of a three-button isn't fully supported, although Opus doesn't really stop you doing it if you work out how to. 
I think there are some other issues with it as well but it's too long since I played around with it to remember.
I guess it's up to GPSoft whether the fix is to stop people from being able to drag menus into three-buttons or to make it a fully supported feature.
Thanks Nudel, I wasn't aware that this configuration wasn't supported.
To be honest I haven't had any other problems with it and figured this was a genuine bug! Each of the menu items can still be edited using a non alt-click method and I reckon the benefits of having a menu in the three button are worth this minor issue.