In another reminder of just how complex the human body is and how many strange things can happen, doctors in Canada cut into a patient to install an arterial line and were startled when dark green blood emerged. Green blood shows up in some invertebrates and, of course, the most famous of TV science fiction characters, the half-Vulcan Mr. Spock from Star Trek.
Spock's blood is supposed to be green because his hemoglobin is based on copper, not iron (which actually would be highly inefficient). In the Canadian case, the 42-year old Earthman on the operating table was suffering from a very rare condition, possibly caused by using too much of the migraine drug sumatriptan, called Sulfhaemoglobinaemia. When this occurs, the red cells stop bonding with oxygen and bond with sulfur compounds instead. It's not fatal, as it resolves itself in the normal turnover of red cells, but it certainly is startling. Look for it to turn up on American television (probably Grey's Anatomy or House) next season.