I was getting this error “xvfb-run: error: Xvfb failed to start” on Ubuntu 14.04, where previously my script had run without problems on Ubuntu 12.04.
My scripts were running calling xvfb-run multiple times, but I was seeing fails due to the error “Server is already active for display 99”
It seemed that the Xvfb wasn’t ending when the xvfb-run command returned.
My solution was to use "xvfb-run -a [mycommand]" so xvfb uses another display if 99 is in use.