Today I learned that there exist systems (presumably misconfigured, but I'm not sure in what way) where the hostname "localhost" does not resolve locally but is sent to some remote DNS server, and then in some cases the DNS server returns a remote address (e.g. a server providing landing pages stuffed with ads).
This was breaking rr, since rr tells gdb to use (e.g.) the command "target extended-remote :1234", and apparently gdb resolves "localhost" to get the address to connect to. I've fixed rr to pass "127.0.0.1" as an explicit local address, but who knows what other software is broken in such a configuration — possibly in insecure ways?