A Foiler is a spoiler that turns out to be false, often deliberately released by the Show. Sometimes this may just be misdirection, when a synopsis is released that omits a twist or major plot point, or deliberately false information may be released. On other occasions, foilers may be unintentional, as for example when a scene written for a casting side, that does not end up in an episode, for example the sides for Bela Talbot.
By the beginning of Season Four, the producers of Supernatural were aware that fans would obtain the sides released for casting, and took extraordinary steps to hide plot points for the season. The casting sides for Castiel indicated he was a demon, and Misha Collins has related that he prepared on that basis for the audition Source. Dean was not named in the sides, although a character called Guy appeared, in order to keep the manner of Dean's reappearance a secret.Read a summary of the sides here
When Genevieve Cortese was cast, Kripke issued a statement saying she would be playing "a small town waitress named Kristy who gets romantically involved with Sam (Jared Padalecki) sometime after Dean’s (Jensen Ackles) death" Source. At Comic Con 2009, Kripke admitted he did this to stir the fans.