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Unless you are running some exclusive site, it is as if you are saying something along the lines of "unless you are wearing brown shoes, you are not welcome into our shop". Now, Versace may be able to do this, but Asda/Walmart couldn't!
Perhaps someone is using a cheap or free firewall that replaces the referer string with the plus symbols so all the header lengths do not need to be changed.
it's the old musicians line "...don't get involved in the politics, man, just play the gig..."
As far as I can determine, Norton Interner Security programs do not do this with the referral strings.
I have to agree Korkus, it should be banned unless someone can produce a program that actually does this. Looks like an exploit attempt to me.