Basic Trope: Someone says something that they believe to be a lie, but it then turns out to be true.
Straight: Fiona claims that Gordon's girlfriend Alex is cheating on him to break them up, and it turns out that Alexandra is cheating.
Exaggerated: Despite it being completely implausible, Fiona swears the strongest oaths she can think of that Alexandra is sleeping with the entire football team, even inventing specific times and dates for it all. Every word turns out to be true.
Downplayed: Fiona tells Gordon that Alex is cheating on him, which is pretty plausible given Alexandra's nature. It turns out that Alexandra really is cheating on him, to the surprise of very few people.
Justified: Fiona has latent Psychic Power, orientated in divination, that she hasn't fully developed (nor that she's aware of... yet).
Inverted: Fiona tries to tell the truth, but is revealed to be incorrect.
Subverted: Fiona tells Gordon that Alexandra is cheating on him, and then breathes a sigh of relief when Alex is found by Gordon in a compromising position. However, Alexandra has a perfectly reasonable explanation for it, and Gordon realizes that Fiona was lying.
Double Subverted: ...until Fiona manages to point out a flaw in Alexandra's story that Gordon overlooked.
Parodied: Fiona feels free to tell whatever lies she likes, confident that as the main character, this trope will save her.
Zig Zagged: Fiona accuses Alexandra of cheating. Gordon and Fiona catch her in a compromising position, but witnesses, and Fiona, claim it's Not What It Looks Like. It later turns out she was cheating, or was she, as the same situation happens again. Finally it's revealed she is, in fact, cheating on Gordon.
Averted: Fiona pretends that Alexandra is cheating on Gordon, but it simply isn't true.
Enforced: The execs have decided that there needs to be a Very Special Episode about lying, but have also decreed that Status Quo Is God. As such, the writers need a plot in which Fiona tells a lie, but nothing comes of it, and this is the only thing they can find — even if it does break the Aesop to have Fiona get away with it.
Lampshaded: "Apparently I'm more honest than I thought."
Invoked: Fiona tells Gordon that Alexandra is cheating on him, and Darren, realizing that Fiona will get caught out, rushes off to seduce Alexandra so that Fiona's accusation comes true.
Defied: Alexandra suspects that Fiona will accuse her of cheating on Gordon, and makes sure she has plenty of witnesses to testify that she isn't.
Discussed: "This isn't a sitcom. If you tell a lie, reality will not magically rearrange itself just to save you from being caught."
Conversed: "Nah, they can't have the main character get caught out in a lie like that. Just watch — they'll build up the drama, and then reveal that it was actually true all along."
Implied: Fiona tells Gordon that Alexandra is cheating on him. When Gordon next sees Alexandra, she is lying outside Kevin's bedroom window unclothed and begins sweating nervously when he greets her. He doesn't question her clearly fabricated story that she has been diving for oysters.
Deconstructed: Fiona deceitfully tells Gordon that Alexandra is cheating on him. Even when it turns out to be true, Gordon stops talking to her due to her untrustworthiness.
Reconstructed: The fact that neither Gordon nor Fiona actually believed Alexandra was cheating on him becomes an In-Joke between them.
Played For Laughs: Fiona suggests Gordon will make a living as a clown. Gilligan Cut to Gordon badly doing an old-timey vaudeville act.
Played For Drama:
Gordon is rendered orphaned because when he comes home one night to find them not waiting for him as anticipated, Fiona suggests his parents were brutally murdered.
Fiona needs to lie to protect Helen, whom she thinks is hiding in her bathroom closet, from a murderous posse. When she tells them Helen is in the attic, they discover her hiding there.
Everybody Fiona knows develops a mortal fear of her, because they know she can say "So-and-So will hit his/her head/contract a terrible disease/break their last pencil," and it will instantly come true.