Many of us have experienced that after updating to Ubuntu Server 12.04 LTS, the halt command seems to do partial job: it really does halt the system, but it does not effectively power it down. Numerous fixes have been issued forums-wide, an official one is still in the works, as this “bug” seems to only effect a select number of systems.
Meanwhile, a quick and dirty fix to power off your Ubuntu 12.04 LTS Server:
# halt -p