Ok yep, I've had 2 hits on that error since disabling the block.
I use the mod which interfaces with SFS (I always forget exactly which one it is - StopSpammer or Bad Behavior), so if anyone - any of these IPs which are hitting the error are in the SFS DBs, and tried to register, they couldn't. And they theoretically could be trying repeatedly to log in.
But to me, that begs the question. If someone is not a member, i.e. a Guest, why would it create an error (for me) and error message (for them). They're essentially just any random person on the internet. Why should that produce an SMF error?
I mean, wouldn't it be more logical for them to see a message saying 'you must register before you can log in' rather than produce an error about the referring url? Shouldn't SMF check to see if they are a member before it spits an error?
And especially if it were bots. There must be bazillions of bots surfing around the internet. Why should they trip an error for SMF if they try and fail to log in?
There must be something I don't understand about the message you referenced.
That means themes other mods anything that has a login form.
How can I find out if a theme has a login form? I thought the themes use the same login form as the default, but it just looks different, to match the theme?