Brachot 34 - Wrong prayers
If one says in his prayer, "You have mercy even on a bird's nest " - they silence him. Some say, because he creates envy between birds and other creatures, but some say this is because he thinks that he explained God's actions as based on mercy, whereas in reality it is not mercy that dictates God's actions, but it is God's actions that define what mercy is.
Likewise, if he says, "We thank You, we thank You" in his prayer, they silence him, since it seems that he prays to two different powers. So too, if he says, "For the goodness that you accomplish on our behalf may Your name be remembered" - that is also wrong, since a man needs to bless God for bad in the same way that he blesses Him for good. Likewise, if he says "The good ones shall bless you," that is also wrong, since the bad ones should be included in the communal prayer together with the good ones.
If one prayed and erred, it is a bad sign for him, and if he is a prayer leader, it is a bad sign for the congregation. However, that is true only if he erred in the first three blessings of praise. Rabbi Chanina ben Dosa would pray for people and then say, "This one will live, and this one will die." They asked him, "How do you know, are you a prophet?" He answered them, "No, but if my prayer flows smoothly on my lips, then I know that it is accepted, and if not, then it is torn up.
Art: Archibald Thorburn - House martins, on a nest beneath the eves