If you add a bunch of abilities that use the same default button position to a single unit, only one of them will be actually visible and castable (it seems that the buttons overlap). It does annoy very much since you can have a lot of abilities that unit could have (e.g. if player chooses the abilities from a vast set or if there are many talents/items/etc for heroes, etc.), but some compositions of the abilities won’t work since some of the abilities will always use the same default button position, and you literally can’t foresee every possible composition.
In Warcraft III, this is solved by letting the abilities to take the closest button slot if the default one is not available. E.g. we have 2 abilities with (2, 0) default button position and add them to a single unit, then the first ability will take the (2, 0) slot, and the second one will take the (2, 1) slot.