Interesting, my problems back then were actually ZMQ-related.
Though to be fair, we were making it a bit complicated by suspending/resuming the OpenVZ containers. From that we sometimes got dead ZMQ connections that were black holing all traffic (incredibly hard to debug - ended up switching because of it).
Yes, most of my crashes on the mongrel2 server I run come from ZeroMQ too. They fix them when I find them, but I actually run my servers under valgrind continually so I can catch them and report them.
OpenVZ is tricky, they are patching the kernel pretty deep and this creates a lot of hard to debug issues. I had a lot of troubles with containers running software mapping big files like MongoDB.
Though to be fair, we were making it a bit complicated by suspending/resuming the OpenVZ containers. From that we sometimes got dead ZMQ connections that were black holing all traffic (incredibly hard to debug - ended up switching because of it).