Alert: could not send password to the passenger helper server: write() failed (32: Broken pipe)
nginx would refuse to start. It wouldn't even write anything to the error log. Argh.
To make a long story short, there were *two* reasons I was getting this error. Here they are, and here are the fixes.
In my nginx.conf, I was specifying the location of the passenger_root and of the passenger_ruby. In both cases, I had an incorrect path due to us having changed versions of ruby and the passenger gem - it had been updated, so its path changed.
We (I) had installed nginx via the command passenger-install-nginx, included with the passenger gem. However, the version of passenger had been updated (from 2.2.5 to 2.2.14, as if it matters) and we (I) had not re-run that install process. I originally didn't think it mattered. However, apparently (and I'm not positive of this) there is a HelperServer that is compiled at install time which points to a particular passenger instance, so if you upgrade passenger, you need to rerun the passenger-install-nginx command. It won't stomp on any configs or anything.
Anyhow, after fixing those problems, whee, nginx and passenger started fine. I hope this helps someone, because I was looking for exactly this post frantically during the Google phase of debugging, and didn't find it. I found pieces of info, but not all of it.