I used to pray, quite often, as a child and teenager. I was raised Roman Catholic, and when I realized that I was gay I prayed for God to make me "normal" and not let me be attracted to men anymore. Hell, I cried myself to sleep over it while praying more nights than I care to remember.
When he didn't answer my prayers, I began to realize that the Christian god doesn't exist.
(In retrospect though, it's a good thing...my life is fabulous!)