Every so often I see page defacement on the Cincom Wiki or on the VW Wiki. There have also been countless attempts to execute server scripts - lots of people have uploaded scripts (seeing the upload capability), and then been disappointed when attempts to execute their scripts fail (Smalltalk based Wikis are pretty secure that way). I just saw a very slick example of Wiki spamming on the Atom Wiki - the wiki being used to discuss/define the Atom posting/syndication format. Have a look here to see an example - the "seo" links - which look normal enough - are actually part of a large scale (100 page +) effort to spam the page. I followed the links; they lead to the site a guy who claims to be able to improve your google page rank (never mind that he just lost any chance of doing business with any of the users of the atom Wiki).
This sort of thing happens on a regular basis on Wikis - heck, the C2 Wiki even has a page devoted to such "hacks" (but I can't find it right now). The main issue with these is the time wasting aspect - someone has to maintain the WIki (clean out dead links, prune pages, etc) - and these attacks make that job more onerous. How do I manage this problem? Well, the CincomSmalltalk Wiki has an RSS Feed. The feed is pretty much just an XMLized version of the recent changes page - but it comes to me instead of me having to go to it. When I see changes pop up, I check them out - and weed out any defacements that have stumbled along. It works pretty well, and it works a lot better than trying to remember to visit "recent Changes" on a daily basis.