A scary thought came to me a few months ago after reading "A Brief History of Time" by Stephen Hawking.. In that book, one of the many interesting things he talks about is "virtual particles". These are atomic or subatomic particles which can suddenly appear as if from nowhere. Apparently, they pop out of another quantum reality. This can happen anywhere from what I read.
The horrible idea that came to me was this: what if several tonnes of anti-matter should suddenly appear in the core of the Sun. Of course, when you have a mixture of matter and antimatter arrived in the same place, they cancel each other out in the form of pure energy. In this case, there would be a massive explosion. Probably not enough to destroy the Sun, this cataclysmic release of energy would send a shock wave towards the surface of our home star. It would probably take a while, due to the way the Sun's form of convection works. When it reached the surface, however...
There would be, at least, an incredibly huge flare which would probably stretch out far into the solar system. I have no doubts that the Earth would get quite a bit of a it. It would probably be a lot like the final few moments in the movie, starring Nicholas Cage, "Knowing". Bye, human race.
Like I have said a few times in my life, "You know you're going to have a bad day if, in the morning, instead of the Sun, something else comes up."