In what ways could a relay be malicious? So we know why to watch out, and possibly what to watch out for.
Discussion
log your IP (dangerous naive users)
link your alt accounts to your "real" account
wasting your bandwith sending you events you didn't ask for
Good list. To best extent clients should guard against what malicious relays may do. For the first two, I know that at least Amethyst got a Tor-routing option.