If you find this in your Apache error logs:
[error] (103)Software caused connection abort: proxy: pass request body failed to 127.0.0.1:50000 (127.0.0.1) [error] proxy: pass request body failed to 127.0.0.1:50000 (127.0.0.1) from 188.8.131.52.1 ()
Make sure you have read the Apache manuals
Also read the "BalancerMember parameters" and "Balancer parameters" sections, e.g. maxattempts.
Relevant settings to test with your application and configuration might be:
SetEnv proxy-sendchunked 1 SetEnv proxy-sendchunks 1 SetEnv downgrade-1.0 1 SetEnv proxy-sendcl 1
It all depends on where Apache is sending the requests and what features the backend supports.
Unicorn doesn't support nokeepalive for a reason, so maybe you should switch to Rainbows!, thin and/or nginx.
If changing the server you use is not an option, you could try:
SetEnv force-proxy-request-1.0 1 SetEnv proxy-nokeepalive 1
BalancerMember xxx keepalive=Off maxattempts=2
If you have issues with large file uploads and unicorn, this might help.