We have created a help information pane which hovers above a button. The help information pane has a DropShadow effect, which is wide enough to cast a shadow onto the button itself. Unfortunately this means, that when the user clicks the button in places which are covered by the shadow, the shadow is clicked, and not the button. This is counterintuitive to end-users - even if the button is in the shadow of the hovering information pane, the user will expect that when clicking the (shadowed) button, the button reacts.

I recommend that by default, all effects should support mouse transparency, to allow click-throughs. I think it is worth considering if mouse transparency should be enabled by default for effects like drop shadow (and others), because in user interfaces in general, clicking the shadow of something, does not have a special meaning, whereas clicking what is placed in the shadow, like the button just mentioned, has a meaning ("activate this button").