Comment 8 for bug 1242215

Sorry, but I have the same problem, am trying the same solution, but I'm getting "E: Version '2:0.123.2189+git35cf912-1' for 'libx264-123' was not found". Is it the wrong repositories?

Specifically:
andrius@Ramblys:/etc/apt$ cat sources.list
#############################################################
################### OFFICIAL UBUNTU REPOS ###################
#############################################################

###### Ubuntu Main Repos
deb http://us.archive.ubuntu.com/ubuntu/ saucy main restricted universe multiverse

###### Ubuntu Update Repos
deb http://us.archive.ubuntu.com/ubuntu/ saucy-security main restricted universe multiverse
deb http://us.archive.ubuntu.com/ubuntu/ saucy-updates main restricted universe multiverse

###### Ubuntu Partner Repo
deb http://archive.canonical.com/ubuntu saucy partner
deb-src http://archive.canonical.com/ubuntu saucy partner

# one more, for DVD stuff. I think
deb http://www.gtlib.gatech.edu/pub/ubuntu saucy main multiverse
andrius@Ramblys:/etc/apt$ sudo apt-get install libx264-123=2:0.123.2189+git35cf912-1
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Version '2:0.123.2189+git35cf912-1' for 'libx264-123' was not found