A few months ago I gave blood at a Red Cross blood drive. A week or so later I got a letter in the mail stating they didn't accept my blood because it tested positive for Hep C antibodies suggesting that I have or have had the virus. The second test they do for Hep C came back negative suggesting that my immune system may have eradicated the virus from my body (happens in ~20% of cases) or it just isn't present in a high enough amount to pick up. Needless to say this really frightened me as I have always been very careful about practicing safe sex and have never done drugs or received a blood transfusion. The Red Cross also included some information about Hep C and that false positives for these tests are possible but very uncommon.  Basically I would have had to have a different virus in my system and the antibodies my body was using to fight it would have to be similarly reactive to the test they use.