In Artemis Fowl: The Arctic Incident, Holly and Root assume it's Artemis behind the B'wa Kell golbin smugglings, but after they kidnap him to find out, it turns out not to be the case.
In the second Night of the Living Dummy book, the heroine, her sister, and her parents enter the sister's room to find that all her bedroom walls have been graffiti'd. The mother yells at the youngest brother, a notorious prankster, but he stops her by saying that this time he's innocent, and that this is messed-up even for him. It turns out that Slappy did it, but who'd believe that?
Harry and Ron use Polyjuice potion Chamber of Secrets to imitate Crabbe and Goyle, Draco Malfoy's two mooks, in the hopes of getting Draco to admit that he's the heir of Slytherin, and thus the cause of all the shenanigans happening at Hogwarts that year. Instead, they hear Draco ranting about how thrilled he is that it's happening and how he'd love to congratulate whoever is actually behind it.
Similarly, in the film version of Harry Potter and the Prisoner of Azkaban, Ron initially guesses when there is a long line towards the Fat Lady that Neville Longbottom must have forgotten the password again, only to learn from Neville's exclamation that he was behind Ron the whole time, and thus couldn't have been the one keeping them out of the common room. It's revealed shortly afterwards that the reason they aren't allowed access is because the Fat Lady was scared away from her painting.
...Because Neville kept forgetting his passwords, made a list, and left that lying around.
In Harry Potter and the Goblet of Fire, Snape blames Harry's trio for stealing the Polyjuice Potion ingredients missing from his stores. Although they were guilty of that 2 books ago, they are not responsible for it this time — Barty Crouch is. Since Snape didn't mention when the ingredients were stolen, Harry thought he was talking about the original theft, so he was simply lying that he wasn't behind it.
Also in The Goblet of Fire, after Filch is again the victim of one of Peeve's practical jokes, he finds Harry's golden egg, recognizes it as something belonging to one of the Triwizard Tournament contestants, and quickly accuses Peeves of stealing. Actually, Harry had simply dropped it while sneaking around with his cloak of invisibility while figuring out a clue. Peeves quickly makes himself scarce, and Snape - who is all-too-often the Straight Man to Filch - convinces the caretaker to forget about it. (Of course, Filch would likely have taken any opportunity to pin something serious on Peeves, given how much of a headache the guy often caused him.)
Fainne says this exact line in Child of the Prophecy, by Juliet Mariller. It's really her grandmother who happens to have similar powers and much more motive...
A variation in Death Masks. Early in the story, one of Marcone's hitters attacks Harry and a priest who's coming to Harry for a job. Later on, when Harry confronts Marcone about it, Marcone blinks in surprise. While Marcone did order the hit, it turns out it wasn't a hit on Harry at all, but on the Denarian-in-disguise priest.
Happens again with Nicodemus in Small Favor. When Nicodemus is talking about the Black Council as an enemy of his, Harry says he already knows that Nicodemus is in cahoots with them, as Hellfire was used in a Black Council attack during Proven Guilty. Nicodemus's reaction is pure shock, as he apparently had no idea about it, indicating that someone in his own faction is betraying him.
Minor character Mortimer Lundquist isn't a recurring villain, but his specialty in magic (talking to ghosts) is considered borderline against the law of magic concerning Necromancy, so the Wardens have spoken to him in the past about not crossing that line. The first few times Harry goes to him for information, Morty starts by saying he hasn't done anything.
Simon Heap in Darke has a hard time convincing Septimus and Sir Hereward that he isn't to blame for the Darke Domaine.
Albrecht Detweiler's centuries-long conspiracy to destroy the Kingdom of Manticore and Republic of Haven — among others — is responsible for pretty much every bad thing that has happened in the history of the two star kingdoms, going back centuries. When the leaders of those two star nations find this out, they're justifiably and incandescently angry and immediately begin making plans to vaporize him into subatomic particles, meanwhile laying any number of charges at his door. Those charges are all very accurate — except for the one about Arnold Giancola.
Benjamin: In fact, they're busy telling the Manty Parliament — and, I'm sure, the Havenite Congress and all the rest of the fucking galaxy! — all about the Mesan plan to conquer the known universe. In fact, you'll be astonished to know that Secretary of State Arnold Giancola was in the nefarious Alignment's pay when he deliberately maneuvered Haven back into shooting at the Manties! Albrecht: What? (blinks in surprise) We didn't have anything to do with that!
Soon I Will Be Invincible: Dr. Impossible has escaped from prison for the thirteenth time and his heroic nemesis Corefire is missing. Could these two incidents possibly be unrelated? As it turns out... yes, much to Dr. Impossible's annoyance.