You might be experiencing a feature of Warcraft that will use an ability bound to a key if you press a similar sequence of keys. For example, say that you have 1 bound to your first hot bar slot, but you do not have any other combination of keys plus 1 bound to anything (i.e. ctrl+1, alt+1, shift+1). If you press ctrl+1, Warcraft will use the ability bound to 1.

So, in your case, if you have number pad 2 assigned as priest heal on one box, and ctrl+number pad 2 assigned to fireball on the mage box when you press ctrl+number pad 2 Warcraft will cast a heal on your priest and fireball on the mage.

To avoid this feature, you have to carefully plan out your bindings.