Wiki checks the content of a post for strings indicative of WikiSpam and refuses to complete the post.
RationaleWikiSpam seemed to be the same URLs splattered over many pages at many times. One reader collected a list of these URLs and suggested I block posts that contained them.
This mechanism was deployed in 2004 with no noticeable effect on spam. It was supplemented with DelayedIndexing which also had little effect immediately. Finally, WikiSpam and other abuses lead to WikiAccessRestricted.
If you get this message, try removing questionable URLs from all parts of the post, not just the part you wrote.