Is my employment in jeopardy?

I have worked in a warehouse for over a year now. The boss told me that I miss some items for truck deliveries almost each day. He told me to relax and that I didn't do anything bad. He had me sign a paper stating that he told me about making errors and that if I didn't understand something I better ask. I feel he gave this paper to a boss that is ahead of him. At the end of the day when we are done he told me Thank You so he must appreciate having me work for him. Should I care about him making me sign this paper and if it continues could I lose the job?