The mysterious suicide of Jacintha Saldanha.
The conspiracy states that this woman, who had no prior history of mental illness, was a generally happy mother of two who simply fell for a dumb prank, didn't actually take her own life but was killed by the British secret service for the embarrassment this prank inflicted on the Royals.
I won't say that I believe it anymore, but the circumstances of this whole account are just so friggin' suspicious.
Also, Princess Diana's death.
The problem with that theory is that the royal family has been humiliated and embaressed far more times and far worse than one prank call which didn't even get played in the UK. If they were going to assassinate people who embaressed the royal family then Prince Andrew would be gone years ago - he starred in Its A Knockout!