The story of Harry Houdini's death — or at least some version of it — is nearly as famous as the man himself. One popular (though not entirely accurate) telling is that Houdini, who would challenge people to punch him in the stomach to show off his pain tolerance and durability, was asleep backstage when someone decided to take him up on that challenge when he least expected it.
The truth is that Houdini was awake for the punches that are believed to have killed him, though he was sitting down because he'd broken his ankle. The challenger, a Montreal college student, delivered not one but five successive hits to his gut. He began to experience stomach pains that night — seems pretty cut and dry, right? It was a stunt gone wrong, an accident. Who would want to murder Harry Houdini? (The answer: Apparently a lot of people.)
Houdini had plenty of enemies. He was a vocal critic of psychics, mediums, and clairvoyants, and he threw his fame and influence behind efforts to expose them as frauds and liars. These efforts included (but weren't limited to) learning their tricks and performing them himself, and also backing a bill that would regulate the profession — he even testified before Congress in support of said bill. Harry Houdini despised self-proclaimed psychics, and the feeling was mutual.
As far as evidence of murder goes, there isn't any. Though in 2008, Houdini's grandnephew George Hardeen attempted to have his body exhumed in an effort to find that evidence. According to his attorney, "There was a motive to murder Harry Houdini, and it was suppressed and covered up" — the attempt was unsuccessful; no exhumation took place.