Looking through the list of recently created tag wikis (10k only) and suggested tag wiki edits, one thing stands out - a lot of the content is copy and pasted directly from other sites on the internet. Case in point:
A Session refers to all the request that a single client makes to a server. A session is specific to the user and for each user a new session is created to track all the request from that user.
Cairo is a software library used to provide a vector graphics-based, device-independent API for software developers.
Copied from cairo's Wikipedia page.
HTML5 video is an element introduced in the HTML5 draft specification for the purpose of playing videos or movies, partially replacing the object element.
Copied from HTML5 Video's Wikipedia page.
Copied from Firebug's Wikipedia page.
A diagram is a two-dimensional geometric symbolic representation of information according to some visualization technique.
Copied from the Wikipedia page for 'diagram'
Others include appworld and recaptcha, both copied from Wikipedia. All of the above were created in the last 24 hours or so on SO. A lot others not mentioned were copied from the software project's homepage without attribution.
The fact is that although we have official guidelines on how to plagiarise Wikipedia, most of the reviewers and editors are not following it (all of the above 'copied from Wikipedia' cases do not contain attribution). And even worse would be cases like the first one, where you are outright copying information from a site with strict ('All Rights Reserved') copyright notices (although I have to admit with Google showing three identical results I can't be sure which is the original).
The solution would have to for both reviewers and editors to be aware of the copyright policy of the sites they are copying from - a notice on the tag wiki edit page warning of copying and the suggested edits review page about rejecting improperly attributed copy-pasted information should do.