Suppose on June 2004, a user read the article "President of the United States", and wanted to cite the information available on the graphical timeline which is shown there.
The recommended way to do so, is to use a "permanent" link, like this:
It is not rare that the content of some tables, timelines, and so on are defined inside of templates.
And this is exactly the case in the above example.
As you can verify, the timeline was added on this change:
which included the content of the first version of the timeline (dated from 27 June 2004):
Now, here is the problem: for someone which reads in 2012 the content of the work which cited that old version of the article, the "permanent link" will show a page from 2004 saying that Barack Obama would be president on 2009!
In other words, old version of the pages transcludes the newer version of the templates, which makes the 2004 version of the article to contain the 2012 version of the timeline, not the one from 2004!
This behavior is bad not only for citations, but also for people trying to navigate through old discussions.
E.g.: if someone is trying to understand why the formatting of some template was changed, or how it evolved to its current form, it is possible he/she will find a discussion in which at some point one editor transcluded the template (when e.g. it had a red text somewhere), and the talk may be completely out of context (because the current version has no red text at all).
(A similar and more difficult problem happens with magic words. E.g.:
shows the revision as of 11:32, 26 May 2009, but it has information which is from 8 March 2012, such as the number of articles, and the software version.
But I don't think this one would be feasible, since the value of these things depend a lot more on the state of the wiki database at a given time.)