How to Fix "symbol lookup error: ... /lib/libavcodec.so.52: undefined symbol: av_gcd" after installation of ffmpeg

It's actually quite easy once you know what's causing the problem. There are two different versions of libavcodec in your system. In my case I installed a fresh ffmpeg from SVN, now I first had to remove the package "libavcodec51". Then everything worked just fine.

sudo apt-get remove libavcodec51

If you still have problems there might be stale libraries in here: /usr/lib/libavcodec.*. Try moving them somewhere else and see if that helps. Otherwise, send me a comment. ;)

See here for a more elegant work-around that tells the package manager we've got newer libraries available.