I would say that the failure to follow the Missal casts doubt on two key qualities for clergy: obedience and truthfulness. When a priest changes the words of the liturgy, he cheats the faithful out of their right to a properly celebrated sacrament. He is selling us short and setting himself up as authority over something which he does not have the power to change. I would question whether such a priest is fit for ministry, even if he seems to have a “wonderful heart” and is a “good shepherd”. If he can’t obey in small matters, how can he be trusted to obey in the large ones? If he cheats the faithful of one right, what other rights will he cheat us on? Paranoid, yes. But I have seen it too many times to be trusting of priests who are unwilling to say the black and do the red.
Now of course the above that I just wrote only pertains to grave abuses of a substantial nature and not at all to the trifle which the OP mentions. So perhaps it is off-topic, but it still needed to be said in light of the previous post.