As for the idea of contaminating something like Mars and having it end up overrun with earth bacteria, I guess it's impossible to prove that it couldn't happen, but I don't it's very likely. Mars is more like the earth than anywhere else in the solar system, but it's still very different. You might be able to find a few organisms here that could potentially survive on Mars, but it's doubtful that any would thrive, particularly to the point of overrunning the planet.
I hope you don't have to eat those words when our giant bat overlords come back down in 30 years.
The perversity of nature is nowhere better demonstrated by the fact that, when exposed to the same atmosphere, bread becomes hard while crackers become soft.