Wikipedia:Wikimedia Foundation error
The common "Wikimedia Foundation error" message (WFEM) is sometimes called a "server error" but might be caused by a recent edit which a user made to the current page being displayed. The message is an indication that the webpage being processed for display could not be completely formatted within the time allotted, typically 60 seconds. It rarely indicates that there is a fatal problem with an actual file server at the Wikimedia Foundation, which has been preparing the webpage.
Template limits cause WFE error
During 2009 into 2012, some very slow templates, when used many times per page, were causing a long delay followed by the Wikimedia Foundation Error box, as follows:
Our servers are currently under maintenance or experiencing a technical problem. This is probably temporary and should be fixed soon.
Please try again in a few minutes.
See the error message at the bottom of this page for more information.
Request: POST https://en.wikipedia.org from 127.0.0.1
Forwarded for: 192.0.2.16
Error: 503, Backend fetch failed at 02:22, Sunday, February 17, 2019 (UTC)
That message gives the impression that the web servers are "too busy" to respond, but often is caused by the complexity of the page that it needs to display. It is quite common to see this after editing a complex article such as Barack Obama. Your edit usually has succeeded, but you don't get to see the result right away. It can be annoying to the editor. This is a known side effect of complicated pages and listed as issue 19262.
In some cases, it might be wise to modify the page to display faster. Since the reader's screen is completely overwritten by that message box, it is necessary for users to realize what page, which they were attempting to read, led to the Wikimedia Error screen. If the page causing the error is changed to use a faster template, then that Wikimedia message page will no longer re-display. Note that a template does not need to be "large" for the WFEM error page to be displayed, but rather, a page that runs very slowly can be the cause. Typically, to be so slow, then an internal template somewhere down the line is being invoked hundreds of times. For more information, see: Help:Template or WP:Advanced template coding.