Operating on the wrong limb

No, this is not a post about social security (thank you, I will be here all week... try the veal). Rather, it is the real deal:

Douglas Hall, 43, went to St. Vincent's Hospital in November 2001 for arthroscopic surgery on his right knee. Despite marking the right knee with an "X," Dr. Andrew Feldman operated on the left knee.

This happens from time to time, but what sort of retard misses his own "X?" I guess he didn't go the full nine yards... an orthopod I shadowed during my community weeks used the "X" method but also wrote a big "NO" on the other leg.