Seems like something went wrong when libproxy was moved from testing to platform. Nothing to worry on your end at the moment, you can safely ignore the pacman warning.
On November 5, Canonical published details about MySQL vulnerabilities for its Ubuntu 12.04 LTS, Ubuntu 11.10, and Ubuntu 10.04 LTS operating systems.
According to Canonical, libproxy could be made to crash or run programs, if it received specially crafted network traffic.
It was discovered that libproxy incorrectly handled certain PAC files.
libproxy: User-assisted execution of arbitrary code