The Mars Curiosity rover recently detected signs of organic compounds on the Red Planet, but NASA won't call the findings definitive. One holdup is the issue of contamination. The trace amounts may be the result of contamination from the rover itself.
The contamination issue could rear its head again should the rover or future expeditions turn up any microbes. Finding microbes on Mars would be a cause for scientific celebration, but a study published in "Proceedings of the National Academy of Sciences of the United States of America" shows that Earth microbes could very well survive in the brutal conditions on Mars.
The study's abstract sums up the issue like so: "The ability of terrestrial microorganisms to grow in the near-surface environment of Mars is of importance to the search for life and protection of that planet from forward contamination by human and robotic exploration."