WARNING: The following packages cannot be authenticated!
  realplay
Install these packages without verification [y/N]? y
Err http://packages.freecontrib.org dapper/non-free realplay 10.0.6.776-1plf1
  404 Not Found
Failed to fetch http://packages.freecontrib.org/ubuntu/plf/pool/dapper/i386/non-free/realplay/realplay_10.0.6.776-1plf1_i386.deb  404 Not Found
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?