Steps to reproduce:

1.) click toolbar button in any app near the edge of the button (but still in clickable area)
2.) don't move mouse at all after clicking
3.) try to click again and it won't register

Oddly enough, if you click the button near the center this no longer happens.

I originally discovered this in solitaire clicking "undo move" repeatedly, but it appears to work in every gtk app.

For example:

1.) open gedit
2.) click save button below the actual "save" label, but still in the clickable area for the button
3.) don't move mouse
4.) alt-c to cancel save dialog
5.) click again and it doesn't register.

Obviously, this isn't a realistic scenario, but any undo/redo type actions are.