Here’s another little sysadmin tidbit to contribute to the internet’s body of knowledge.
If you’re setting up a Subversion (svn) server and get an error like this when trying to commit to it:
svn: Commit failed (details follow): svn: MKACTIVITY of '/!svn/act/[big UUID]': Could not parse response status line. (http://whatever)
…it’s often an unhelpful way of telling you that Subversion servers don’t work behind reverse-proxies, such as Squid.
In this case, I had Subversion running through Apache on port 80 with Squid in reverse-proxy HTTP-accelerator mode in front of all Apache-port-80 traffic.
Moving it to a non-proxied port (81, 8080, whatever) fixes this problem.