In Heroes-Inc., the first mission the titular agency partakes in is rescuing Princess Peach. They immediately consider Bowser as a prime subject, but quickly rule him out because of his "air-tight alibi". They later find out that Peach actually is in Bowser's castle, but it is a robot. The actual Big Bad planted the robot in the castle so that when the heroes rescued it, it would easily be in a position to assassinate Mario.
In Homestuck, Terezi automatically pins the murders of Feferi and Kanaya on Vriska, although even she admits this doesn't make much sense and is largely due to her own fixation on her. When she confronts Vriska, the latter is adamant that she is only guilty of Tavros's death.
In Drowtales, it's immediately assumed that Kiel'ndia assassinated Shimi'lande, the Kyorl'solenurn Ill'haress since she had very publicly assassinated another figure, the Val'Sharess or more accurately her Body Double since the real one has been dead for decades and a figure resembling her was seen at the scene. Despite this Chirinide, who has her own complicated history with Kiel, expresses doubt at this version of events, and later Kiel herself denies she had anything to do with it, not that anyone listens.