Logout is NOT broken in OS X Lion either in Chrome (14.0.835.163) or Firefox (6.0), on my machine at least.
However, logout can be extremely slow, and it seems to get slower if you bang on login/logout a lot; HN might have coded in some delays to keep users from slowing the server down.
My guess is, this user just didn't wait long enough for logout to work.
If it's hanging in both browsers, could be a network issue... I've occasionally seen paths to some (but not all) websites suffer from hanging HTTP/TCP connections, perhaps due to packet sizing/fragmentation/loss issues.
Does the problem happen for you at another location? Any new local routers/switches/access-points in the local mix? Does another OS on the same network (perhaps even via a VM on OSX) show the same issue?
Notably, when testing this, I discovered that logging out of HN from anywhere logs you out from everywhere. So even using a smartphone to login/logout might be a workaround for getting your OSX login session invalidated. (Of course discarding the browser cookies should work as well for the local browser.)
However, logout can be extremely slow, and it seems to get slower if you bang on login/logout a lot; HN might have coded in some delays to keep users from slowing the server down.
My guess is, this user just didn't wait long enough for logout to work.