In WWDC2013 Session 702: Efficient Design With XPC, Apple's engineer said this:

They said, if you see any message in system log like this for libxpc:

assertion failed: 13A476z: libxpc.dylib + 2794 [some UUID value]: 0x11

Please fire a bug report.

If this is not translated into text, people won't be able to find this! So I put this here, and see if this would help anyone. I haven't encountered this kind of error, but it might help someone else who is googling over and try to find something useful.