Community-testing repo
Yes, you heard well.
I’ve seen a lot of guys on forum and on irc who had problems because of libpng/libjpeg thingy we have now in testing.
If you are a testing user, don’t forget to enable community-testing as well. The repo is available in pacman.conf, you just have to uncomment it.
I did have that issue, Xorg server started and after few millisecond it halted. I had to restart on init 3 and disable [*testing] repos, so I could downgrade everything ( pacman -Suu ).