Bizarre. I was doing some filesystem cache experiments on a client machine yesterday. It shouldn't be possible for an out-of-spec client to confuse the server enough to interfere with other client sessions, but that might be what happened. I've restarted the builders and I'll keep and eye on them.
The restarted builders each succeeded with make.rc and then failed to write back the snapshot with context time exceeded. I wonder if the timeout deadline is long enough to allow for several builders contending for a domestic ADSL connection?