Sometime means "at an indefinite or unstated time in the future."
Some time means "a period of time."
Sometimes means "occasionally, now and then."
Examples:
"Why don't you come up sometime and see me?"
(Mae West in She Done Him Wrong, 1933)
"You must give some time to your fellow men. Even if it's a little thing, do something for others--something for which you get no pay but the privilege of doing it."
(Albert Schweitzer)
"I am so clever that sometimes I don't understand a single word of what I am saying."
(Oscar Wilde)
Sumber: http://grammar.about.com/od/words/a/sometimeusage.htm