I am scared that i will be fired?

Okay, this week I was up for review for my progress at work. Upper management pulled two of my cases and told my boss that I lack documentation about a situation that happened. My boss tried to defend me and stated that those two cases were NOT mine at the time that an incident happened and that the previous worker should have documented what happened. He and I were told that it does not matter, that I am at fault about what happened because the cases are now mine. I went back to talk with the person from upper management this evening to explain what happened and asked what could I have done differently. She explained that I should have documented the incident when it happened, but once again I tried to explain that I DID NOT have those cases then, matter of fact, i was not even working at this job when this incident occurred. I left her office feeling shocked and angry. I have being doing well with everything else at my job, but I feel that this will count against me. Now I have to go through ALL my cases and make sure all things are properly documented. The situation in the cases happened in the past and was out of my control, but I was still blamed.