Wikipedia is not a reliable source for citations elsewhere on Wikipedia, or as a source for copying or translating content. As a user-generated source, it can be edited by anyone at any time, and any information it contains at a particular time could be vandalism, a work in progress, or simply incorrect. Biographies of living persons, subjects that happen to be in the news, and politically or culturally contentious topics are especially vulnerable to these issues. Edits on Wikipedia that are in error may eventually be fixed. However, because Wikipedia is a volunteer-run project, it cannot constantly monitor every contribution. There are many errors that remain unnoticed for hours, days, weeks, months, or even years (see Wikipedia:List of hoaxes on Wikipedia). Additionally, it is possible that some errors may never be fixed. It is also possible for an edit correcting an error to later be reverted. Therefore, Wikipedia should not be considered a definitive source in and of itself. This includes articles, non-article pages, The Signpost, and non-English Wikipedias.
Wikipedia pages often cite reliable secondary sources that vet data from primary sources. If the information on another Wikipedia page (which you want to cite as the source) has a primary or secondary source, you should be able to cite that primary or secondary source and eliminate the middleman (or "middle-page" in this case).
Always be careful of what you read: it might not be consistently accurate.
An exception to this is when Wikipedia is being discussed in an article, which may cite an article, guideline, discussion, statistic or other content from Wikipedia or a sister project as a primary source to support a statement about Wikipedia (while avoiding undue emphasis on Wikipedia's role or views and inappropriate self-referencing).
Articles are only as good as the editors who have been editing them—their interests, biases, education, and background—and the efforts they have put into a particular topic or article. Since we try to avoid original research, a particular article may only be as good as (a) the available and discovered reliable sources, and (b) the subject may allow. Since the vast majority of editors are anonymous, you have only their editing history and their user pages as benchmarks. Of course, Wikipedia makes no representation as to their truth. Further, Wikipedia is collaborative by nature, and individual articles may be the work of one or many contributors over varying periods. Articles vary in quality and content, widely and unevenly, and also depending on the quality of sources (and their writers, editors, and publishers) that are referenced and/or linked. Circumstances may have changed since the edits were added.
Occasionally, inexperienced editors may unintentionally cite the Wikipedia article about a publication instead of the publication itself; in these cases, fix the citation instead of removing it. Although citing Wikipedia as a source is against policy, content can be copied between articles with proper attribution; see WP:COPYWITHIN for instructions.