I don't remember this ever happening in the past, but now say for example i summon Durnehviir, then conjure a Daedra Lord, the Daedra Lord will overwrite Durnehviir and remove him, leaving me with a huge cooldown on dragon shouting and leaving me with nothing to show for it... Same goes for any other shouts that summon something...
Needless to say, this is balls.