Debian's Apache 2.2.3
With the old Apache 2.0.x, I'd sometimes see proxy errors if my JBoss server took too long to respond. It was not reliably reproducible, but it looked a lot like Apache Bug #37770 around keep-alive connections through the proxy.
Apache 2.2.3 recently hit Debian, so when I got it installed, I was pleased to see that they seem to have included the patch for this fix, and I no longer see the proxy errors. My previous approach to avoiding the issue is just excessively tuning JBoss, but that didn't even eliminate all the failures.