Several people I've met play Settlers of Catan with the house rule "you may not build anything that isn't a knight next to the robber". As far as I can see, it's not in the rule book.
To clarify: this means that you can't place a road, settlement or city next to the robber. When we play, city improvements and the merchant are unaffected. On the rare occasion we combine with seafarers, we say the robber prevents land building, and the pirate prevents ship building/movement.
I actually prefer this variant, but this question isn't about that. My question is:
Where did this rule come from? Was it in an earlier version of the rules, or was it used for some tournaments? I've met a number of unrelated players (in Australia/Germany) who have heard of this rule, so I wondered if it came from somewhere other than word of mouth.
I'll also accept "this isn't in any of the published rule books and isn't used for tournaments" if that is the case (I only have the latest rule book to hand, so I can't check).