This spelling error comes up pretty often, and it seems odd: “pasttime” for “pastime”. I am thinking, this word probably is a compound of “pass” and “time”, so “passtime” I would have no trouble with. How do people come up with “past” + “time”? It makes no sense, unless you think a pastime is some leisure activity from days of yore.
Today’s instance of this infraction is at Culture 11.