Once in a while it happens that a Jenkins build fails because the Xvfb it wants to create already exists:

Xvfb starting$ Xvfb :1 -screen 0 1024x768x8 -fbdir /u10/app/gcadmin/jenkins/stable/jenkins_data/2014-07-24_11-54-567539530762307179860xvfb
_XSERVTransSocketUNIXCreateListener: ...SocketCreateListener() failed
_XSERVTransMakeAllCOTSServerListeners: server already running

Fatal server error:
Cannot establish any listening sockets - Make sure an X server isn't already running

Settings are pretty default: I use the number of the executor with offset 1 (so it is not possible for another server running with this number). The workaaround at the moment is manual deletion from time to time.