The Creepy Candy Coating Corollary - S3-E5
Corrected entry: Wil Wheaton tells Sheldon that he missed the convention because his grandmother died. When Wheaton later refers to a living grandmother, Sheldon realizes he has been tricked. He shouldn't - presumably Wheaton, like everyone else, has two grandmothers (mother's and father's side) and thus Wheaton announcing he has a living grandmother in no way contradicts what he said before. This alone shouldn't tip Sheldon off that Wheaton was lying.
Correction: In context of how the scenes played out, it is directly implied (and very obvious) that Wheaton is talking about the same grandmother.
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