In the portion of Vaeschanan, Moses prays to enter into the Land. The word Vaeschanan is the numerology of 515, indicating that Moses prayed 515 prayers to nullify the Divine decree against his entry into the Holy Land. Finally, G-d told him: Enough! Do not pray any more. Why did G-d not want him to pray any longer?
Says the Munkatcher Rebbe (in his commentary Chaim Vesholom), because G-d knew, that with one more prayer, Moses would have broken the decree! So G-d told him, stop praying.
And the Munkathcer concludes: Sometimes Jews get tired of praying for Moshiach and redemption. They should learn from this that it is a mistaken approach. The power and potency of prayer is unbelievable.